3. 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
Because the verb phrases used to describe the bats' duties are governed by the phrase different duties such as, they should each be expressed in the present participial (or "-ing") form to parallel defending and scouting. Choices A, C, D, and E all violate parallelism by employing infinitives (to...) in place of participial phrases. In E the singular sentinel is not consistent with residents，and the omission of and distorts the meaning of the original. Only B, the best answer, preserves the sense of the original, uses the correct idiom, and observes the parallelism required among and within the three main verb phrases.
做对了，在对E项的解释方面请大家帮我理解，谢谢 | 
