[讨论]关于菜鸟手册对og3的解释

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(记住这种形式的单数复数一致---代词指代不一致的一种变体), 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.

以上是菜鸟手册对这道题的解释。我不理解灰色部分的含义（这里存在单数复数一致的语法点吗？），有哪位nn 可以解释一下？

楼上MM，这里的确存在单复数一致的问题，因为从主语Some bat caves,看到同位语 like honeybee hives, 到宾语have residents 都是复数对应形式，再到后面充当哨兵也理所当然要用复数啦！ acting as sentinels ，因为总不能说一群东东充当一个哨兵吧！

举个简单的例子：Many students hope to become a movie star when they grow up! 就有问题吧，不能说许多学生长大了都想成为一个明星吧（中文角度听起来好像还OK,奇怪，呵呵，anyway)！要当也是当一群明星嘛！

谢谢！我懂了！