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
Parallelism + Agreement
The original sentence has an error in parallel structure．It starts by using the-ing (participial) form to list the bats’ duties．Defending，acting，and scouting all use the same-ing form．The phrase to sound uses the to-(infinitive)form，and so it is not parallel．The word sounding is required in this sentence
A To sound is not parallel to defending, acting，and scouting．
B Correct．This sentence has sounding, which properly parallels defending, acting, and scouting．
C To act and sound are not parallel to defending and scouting．
D To act and to sound are not parallel to defending and scouting．
E Introduces agreement error(a sentinel)；to act is not parallel to scouting．
The correct answer is B．
这道题目应该是DEFENDING ,ACTING ,SOUNGDING,SCOUTING这四个要平行,对吗?
为什么有两个AND呢? | 
