干簧管通过（）来驱动，构成干簧感应器，用以反映位置信号。

A、铁块

B、永磁体

C、非金属体

D、光器件

The survey shows that urban students spend about twice ___ money the ones from rural areas do.

A．the amount of

B．more than

C．as more as

D．as much as