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

1.  平行结构：
此题大平行里小平行
A．平行结构的形式对称 ==》现在分词和不定式不对称。
B.  平行结构可以有多个层次，此处就是  such as A, B1 and B2, and C1.  其中 A，Ｂ，Ｃ大平行， B1 and B2;  是小平行。

2.  单复数一致：
Residents that … acting as + 复数名词对应，表示同一内涵的对象单复数一致。

3.  保持原句意思
平行结构改成分词修饰结构会改变原句的意思。

4.  Such as / Like 用法：
A like B.      A 和 B是同类的事物进行比较。
A，such as B , C          B 和Ｃ是对Ａ所包含事物的列举。