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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(前后不一致)

B.     As the cost of wireless service plummeted in the last year and as mobile phones became increasingly common(前后不一致plummepted不是谓语动词，而后面的became 是动词), 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（总产生疑问）

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（前后也不一致,去年是现在完成时态然而后面是现在进行时态）, many people are

B中的plummeted为什么是非谓语动词呀？