OG-3

3.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

Because the verb phrases used to describe the bats' duties are governed by the phrase different duties such as, they should each be expressed in the present participial (or "-ing") form to parallel defending and scouting. Choices A, C, D, and E all violate parallelism by employing infinitives (to...) in place of participial phrases. In E the singular sentinel is not consistent with residents, and the omission of and distorts the meaning of the original. Only B, the best answer, preserves the sense of the original, uses the correct idiom, and observes the parallel­ism required among and within the three main verb phrases.

为何是such doing sth, doing as sth and doing sth, and doing sth and dong sth , 第二个doing,即acting前为何没有and,这不是也不平衡吗？请nn们指教

谢谢斑竹！

不过今天仔细看了一下，好像不是这样的吧，defending the entrance, acting as sentinels and sounding a warning at the approach of danger, and scouting ……，这三个doing是并列的吧，不像斑竹说的acting and sounding……是修饰the entrance的。我搞不明白sounding在这里到底是做什么的，和哪个并列，如果是和acting并列的话，就有上面体的那个问题了：为何是such doing sth, doing as sth and doing sth, and doing sth and doing sth , 第二个doing,即acting前为何没有and,这不是也不平衡吗？

版主说得很对。

acting... and sounding ...是修饰defending的。acting as dentinels and sounding a warning都是defending the entrance的手段。

这样解释关于and的问题也能说通了，acting 和sounding 平行，defending 和scouting平行。

明白了，对不起斑竹，是我自己想错了，哈哈。

不愧是版主nn.

谢谢andrew

这样就很清楚了，哈哈