This is a test of list parallelism, made tricky by mixing up with pseudo parallels
Th basic theory of list parallelism stipulates that it is important to identify the elements of the series first. Within each series, the last factor is always preceded by the word “and” which marks the end of the series.
If there are more than one series, then the last series by itself has to be again preceded by another “and”.
If there are more than one series in a passage, ensure that each of the series maintains its own parallel structure within its domain.
In the given text, we have two series.
1. One is the ‘defending’ series that includes acting as sentinels and sounding a warning
2. The other is the ‘scouting’ series that includes searching for new food and roosting
These two arms must be separated by an ‘and’; all choices do that and so no issue about it.
But it is the parallelism within each series that is presenting a problem here. As you may see,
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 --- acting and to sound is not parallel in the first series.
(B) acting as sentinels and sounding – perfect parallelism -- correct choice
(C) to act as sentinels and sound --- The infinitive spoils the parallelism since, the first duty is marked as defending and the other duties must also be in the same ing form rather than in infinitive form
(D) to act as sentinels and to sound --- same as C
(E) To act as a sentinel sounding --- same as C and D
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