求助语法笔记的一道题

schlaf

175.(GWD-9-Q3)Because of wireless service costs plummeting in the last year, and as mobile phones are increasingly common, many people now using their mobile phones to make calls across a wide region at night and on weekends, when numerous wireless companies provide unlimited airtime for a relatively small monthly fee.

A.Because of wireless service costs plummeting in the last year, and as mobile phones are increasingly common, many people

B.As the cost of wireless service plummeted in the last year and as mobile phones became increasingly common, many people

C.In the last year, with the cost of wireless service plummeting, and mobile phones have become increasingly common, there are many people

D.With the cost of wireless service plummeting in the last year and mobile phones becoming increasingly common, many people are(D)

E.While the cost of wireless service has plummeted in the last year and mobile phones are increasingly common, many people are

这里的D选择种为什么要用plummeting呢？是省略了have been 么？

chasingM7

独立主格结构吧。plummeting 和 becoming平行。