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

Can anyone explain to me the answer? detailing parallelism??

OA B