Though he had failed twice, the strong-willed boy decided to make a third attempt_________ it. [ ]

A.to        

B.on        

C.at        

D.B &C

Modem是计算机通过电话线接入Internet时所必需的硬件，它的功能是______。

A．只将数字信号转换为模拟信号
B．只将模拟信号转换为数字信号
C．为了在上网的同时能打电话
D．将模拟信号和数字信号互相转换