Some bat caves, like honeybee hives, have residents that take on different duties such as defending the entrance, acting as sentinels and to sound a warning at the approach of danger, and scouting outside the cave for new food and roosting sites.
(A) acting as sentinels and to sound
(B) acting as sentinels and sounding
(C) to act as sentinels and sound
(D) to act as sentinels and to sound
(E) to act as a sentinel sounding

Parallelism. C,D and e are out....'to act as...'
A is also wrong ...'to sound..'
B it is.

Hi Darmody,

Some bat caves, like honeybee hives, have residents that take on different duties such as defending the entrance, acting as sentinels and to sound a warning at the approach of danger, and scouting outside the cave for new food and roosting sites.

(A) acting as sentinels and to sound

Changes the meaning by changing the list

(B) acting as sentinels and sounding

Correct, maintains parallel structure in all the lists

(C) to act as sentinels and sound

Changes the meaning; sentence now implies that they defend to act as sentinels and sound...

(D) to act as sentinels and to sound

Same as C

(E) to act as a sentinel sounding

Again changes the meaning; moreover "sentinel" should be plural as "residents" is plural.

The correct answer choice (B) presents the list with parallel structures. I have marked the lists by brackets in the correct version of the sentence.

Some bat caves, like honeybee hives, have residents that take on different duties such as (defending the entrance, acting as (sentinels and sounding a warning at the approach of danger), and scouting outside the cave (for new food and roosting sites)).

The best way to find out the lists is by the meaning differences between them.

for example

In sports I like ((football, baseball and table tennis), and in music I like (Rock, Jazz and Classical)).

In GMAT whenever you see "and" you should check for the logical lists and make sure the structures being listed are parallel grammatically.

Hope it helps,

Vercules
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Hi Darmody,

I can understand your concern. The correct answer choice is not a well constructed sentence and looks a little weird. But, it is the least bad among all the answer choices. The other choices are clearly wrong because almost all of them are changing the intended meaning of the sentence.

If you break the sentence; defending the entrance, acting as sentinels and defending the entrance, sounding a warning at approach of danger.

ie. by acting and sounding they are defending the entrance.

Consider an example similar in structure:

John is defending his position, acting as a lawyer and presenting a sound argument that he is not guilty.

John is defending his position in two ways

i) by acting as a lawyer
ii) by presenting a sound argument

One more similar example

By doing his job properly,acting as a role model and creating a conducive environment for the whole team, John has become the most admired manager in the company.

Tip: Concentrate more on errors in sentence correction; it is far more easier to prove that a choice is incorrect than to prove that it is correct. In your practice try to get familiar with the error patterns that sentence correction questions test.

Moreover, SC 1000 is not a very reliable source for practice. First it is quite old and contains short questions like this one, while in these days longer sentences are seen in SC in GMAT, second most of the questions are good but the bad ones could really screw up your concepts. Do it if you have exhausted all the official stuff at your disposal.

Hope it helps,

Vercules
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