Sentence Correction

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

oa b

[spoiler]im totally confused, y are we using "ing" here[/spoiler]

the question is related to parallelism..
since the non underlined portion is in ing form...
we need to make the other parts parallel...
and b is the right option...