OG verbal 2nd edition 第二题，两个and 连用怎么理解呢？

小猪快跑莉

Q: 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
A: CORRECT B
could  somebody explain the parallel construction A, B and C, and D ? Isn't it a little bit awkward and wrong ? In which case can we have a list with two "and" ?

Thx !       

yuanhaowen

acting as sentinels and sounding a warning at the approach of danger
这是大平行里面的小平行，你需要根据逻辑意思判断，这才是gmat的根本

小猪快跑莉

yuanhaowen 发表于 2014-7-26 15:52
acting as sentinels and sounding a warning at the approach of danger
这是大平行里面的小平行，你需要 ...

谢谢亲的解答。。一开始的理解是1.大平行：defending the entrance, and scouting outside the cave for new food and roosting sites..2.第二层平行是：acting as sentinels and sounding a warning at the approach of  danger( 解释怎样defending the entrance) 但是OG对其的解释是，“This sentence has sounding ,which properly parallels defending, acting and scouting ". It seems to indicate that the four participles are equally parallel..然后我就有点迷茫了。。