题目选项分析

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

As the cost of wireless service plummeted in the last year and as mobile phones became 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.