[求助]GWD9-2答案为什么不一致呢？

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

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

[求助]GWD9-2答案为什么不一致呢？到底应该是D还是E呢？能帮忙解释一下么？我觉得两个好像都麽问题~~

谢谢NN哦！！

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

E 有几个问题： 1）Last Year, 所以要过去式，2）句子意思里有因果关系的存在，而E用WHILE不能表达这个意思，