一道SAT改错题

问题描述:

一道SAT改错题
Representatives of 2 independent groups (have been asked) to rewiew resent studies comparing the (monthly) costs of using cell phones with (conventional phones).(no error)
conventional phones应该改成the monthly costs of using conventional phones 还是using conventional phones?

the monthly costs of using conventional phones
因为是价格的对比 前后要一致