tusk is a singular count noun and it does not have a determiner (a, the, my, some, John's, 1, 2, 3, 4, etc.).

We need a determiner for every singular count noun. This is one of the most common mistakes--we need to look at every underlined noun and figure out whether it is a count or non-count noun. If it is a count noun, is singular and does not have a determiner, then it is wrong. You have a big clue here because of the word single which means that tusk is singular and countable. Did you choose having tusks? This is correct because it is reduced from that have tusks.

This is a word form question. This is one of the top five problem areas! implication is a noun; we need a verb, implies. If you were confused by While their, you should remember that while = although.

Many people forget this rule. We need a possessive subordinating conjunction, whose here. Here's a simple test: can we say sperm whale's name? If we can use the 's form here (the possessive form), then we should use whose and not which.

Finally, we need to say that the name comes from something, not comes something since come cannot be used with another noun after it, i.e., come is intransitive and cannot take an object. 

First of all, we need a determiner for the singular count noun cavity. We need a determiner for every countable noun. This is one of the most common mistakes--we need to look at every underlined noun and figure out whether it is a count or non-count noun. If it is a count noun, is singular and does not have a determiner, then it is wrong. That means we should choose C or D. Remember, TOEFL doesn't test the difference between a and the, so we don't have to worry about whether a or the is better. Finally, D is better because we should use an adjective clause here, that contains. We should use for containing to explain a purpose, for example Copper is one of the best metals for cooking.

This is basically another word form question and a meaning question together. All of the answers fit into certain grammar patterns, but only A has the correct meaning. This question is difficult also because cosmetic can be both a noun and an adjective. For example, in B, cosmetic is a noun and in C cosmetic is an adjective. We want to say that the solid is used in cosmetics, as in A. B says that the solid is the cosmetic (noun). C says that the solid is cosmetic (adjective). Finally, we should remember that chief means main and chiefly means mainly. If this question was hard for you, don't worry--most TOEFL questions depend on grammar and not on meaning! 

This is a preposition question. This is one of the top five problem areas! We need to say that the liquid is used as a lubricant. used for can be used when we want to tell the purpose; for example, We should not use a computer for a simple task.

We need a main subject and a main verb here. This sentence actually has two sentences--Canaries used to be used in mines to detect odorless, colorless, and poisonous gas is the first sentence and when the canary died, it meant that poisonous gas was present is the second sentence, so we need to choose B. If we choose D, we don't know what it refers to--? We have no referent for the pronoun: the poisonous gas meant what was present??

We have a reduced adjective clause here--that were once thought to be indicative of larynxes is reduced to  once thought to be indicative of larynxes. We should remember that when we reduce an adjective clause, we keep the adverb. This question is hard because the grammar and vocabulary is pretty advanced.

head is a singular count noun. We need a determiner for every singular count noun or we need to make the singular count noun a plural noun. This is one of the most common mistakes--we need to look at every underlined noun and figure out whether it is a count or non-count noun. If it is a count noun, is singular and does not have a determiner, then it is wrong. In this case, we need to change head to heads. We have some other clues here as well--whales, are, and members all tell you that we are talking about many whales and we should also use the plural form of head.

Correction: known as. We need to say known as. In general, known as should be used to show that we can use a different name for something, for example, Matsuda, the giant Japanese automaker is known as Mazda in the United States. We should use known for when we want to talk about the reputation that something has, for example, The state of Bahia in Brazil is known for its delicious fish stews and beautiful music.

We can use an adjective clause here: a cavity that holds sperm oil. Each of the other answers contains a mistake. In A, to holding is incorrect; we should say to hold. In B, held is wrong; we should use present tense and we also need to use that. Finally, D is wrong because we should not say a sperm oil since oil in a non-count noun and should not be used with a.

First of all, we need a main verb since the main subject is progress and there is no verb in the sentence. That leaves us with A and D. A is wrong for two reasons. First, the easiest thing to explain, is that we should use the present perfect, has been, for progress here because we can infer from this sentence that this progress is something that has been occurring for a long time and is still occurring in the present. 

The second reason A is wrong is the placement of the adverb largely. Rules for adverbs are difficult in English! In this case, we need the adverb of frequency between the auxiliary verb has and the past participle been. One way to help we can remember this is to remember a simple sentence with an adverb of frequency, for example, My mother has always told me to work hard. If we can remember this sentence, we can remember the rule for adverbs of frequency.

You need an article for every countable noun. 

area is a singular count noun and it does not have a determiner (a, the, my, some, John's, 1, 2, 3, 4, etc.).

We need a determiner for every singular count noun or we need to make it plural. This is one of the most common mistakes--we need to look at every underlined noun and figure out whether it is a count or non-count noun. If it is a count noun, is singular and does not have a determiner, then it is wrong.

Price is a singular count noun and it does not have a determiner (a, the, my, some, John's, 1, 2, 3, 4, etc.).

We need a determiner for every singular count noun or we need to make it plural. This is one of the most common mistakes--we need to look at every underlined noun and figure out whether it is a count or non-count noun. If it is a count noun, is singular and does not have a determiner, then it is wrong.

This is another top five problem area! Here we have a reduced adjective clause: hooves that are more developed reduces to hooves more developed. Also, we can't say more developing because it would mean hooves that are more developing. We should use more with the adjective developed to show the degree. If we wanted to use a comparative with developing, we would have to use a sentence like this: Many businesses in Asia and Latin America are developing more quickly than those in  Europe and North America.

In this sentence, we need a subordinate subject for the verb starts. Also, we have a reduced adjective clause: the water that is contained inside reduces to the water contained inside.

nickname is a singular count noun and it does not have a determiner (a, the, my, some, John's, 1, 2, 3, 4, etc.).

We need a determiner for every singular count noun or we need to make it plural. This is one of the most common mistakes--we need to look at every underlined noun and figure out whether it is a count or non-count noun. If it is a count noun, is singular and does not have a determiner, then it is wrong.

earn takes two objects--a direct object and an indirect object. In this sentence, the main subject is loyalty, the direct object is the nickname, and the indirect object is them.

Essentially, this is a reduced adjective clause: the homing pigeon, which was once abundant, is now extinct reduces to the homing pigeon, once abundant, is now extinct which, in turn, can be changed to once abundant, the homing pigeon is now extinct. Remember, when we reduce an adjective clause, we keep the adverb.

Essentially, this is a reduced adjective clause: hydrogen, which composes over 99% of the studied universe, is by far the most abundant element in the known universe reduces to hydrogen, composing over 99% of the studied universe, is by far the most abundant element in the known universe which, in turn, can be changed to Composing over 99% of the studied universe, hydrogen is by far the most abundant element in the known universe.