1. 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
why B?
if it is B, then " defending..., acting as ... and sounding ..., and scouting ..."
won't the "and" before sounding cause problem?
such as defending the entrance, and scouting outside the cave for new food and roosting sites，是并列的；保卫入口，侦察外围
acting as sentinels and sounding
a warning at the approach of danger，是并列的，担当卫兵，且危险来临时发出信号
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