GMAT题求解
求最正确的一个选项并详解
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.
划线部分是：acting as sentinels and to sound
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