几道紫皮的语法

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 foo and roosting sites.

B acting as sentinels and sounding

C to act an sentinels and sound

D to act an sentinels and to sound

E to act as a sentinel sounding

答案选的是A，我知道是平行结构，但是如果都是-ing形式的平行结构，不是应该在最后一个连接的时候才用AND 么？为什么这里的AND 不是啊？我还以为只有defending 和 scouting平行呢。请求NN指点。

2、In recent years cattle breeders have increasingly used crossbreeding, in part that their steers should acquire certain characteristics and partly bacause crossbreeding is said to provide hybrid vigor.

B in part for the acquisition of certain characteristics in their steers

C partly because of their steers acquiring certain characteristics

D  partly bacause certain characteristics should be acquired by their steers

E partly to acquire certain characteristics in their steers

这个题目我觉得要是与后面的结构对称的话应该都用partly because 才对啊，为什么答案选择E呢？

1，第一个and 是局部结构中的连词，总的大平行结构是明确的。

2，to acquire 主动语态配搭前面  cattle breeders have increasingly used crossbreeding 的表述，比certain characteristics should be acquired 的被动语态更优先