2. ______ are most mammals able to receive the most information about what is happening in their immediate environment.

Because they use sight
By means of sight

This one's tricky! We need to notice that we have subject-verb inversion, so we must use a structure that requires inversion.

With the sights
Using sight

3. The Consumer Price Index in the United States ______ the inflation rate.

as the primary indicates
is the indication primarily
primarily indicates of
is a primary indicator of

We need a few things: A main verb, correct word order, and correct word form.

 4. Some of the most influential works in twentieth-century American classical music were composed in the period between the first and the fifth decade.

works
were
and
decade ==> decades

We are talking about two different decades here, the first one and the fifth one.

5. Although the grammar of the English language is chiefly derived ______ Germanic languages, some 80% of its vocabulary comes from Latin.

for
from

Memorize this! derive + from

with
of

6. On the land or in the sea, carrion creatures have adapted to feed by any type of food that is available in their habitat.

On
have adapted
by

Memorize: feed + on. We should also know the meaning of this word since it appears fairly frequently on the TOEFL: feed on = eat

is available

7. The brightest point seen in the night sky is typically not a star but Venus, it is the second planet from the Sun.

seen in
typically
it is ==> it is (get rid of it!) or which is

Two main sentences with no conjunction to join them.

the Sun

8. Information technology is quickly becoming the most important economic export ______.

in certain geographical areas

Two things: Correct preposition and word order.

for area that are geographic
in certain areas geographical
of certain geographical area

9. Although originated in tropical Africa, watermelon is now cultivated all over the world.

Although
originated ==> originating

The perennial problem: -ing or -ed form??? In this case, we need the -ing form since watermelon is the subject of the verbal originating.

is now
all over

10. In 1995, Newt Gingrich was sworn ______ Speaker of the House, the first Republican Speaker in more than 40 years.

in as

More than 90% of all people who have tried this question have answered it incorrectly, so I'd say this one's pretty darned tricky.

To choose the right answer, we should know the expression to swear somebody in, which refers to the ceremony in which a person (for example, a new president) gives an oath promising that s/he will do a great job! Then, we need to know that we should use the preposition as if we want to mention the position the person is taking. Finally, we need to know how to chance this to passive voice.

to be
to
in