The first electric lamp had two carbon rods from which vapor served to conduct the current across the gap.

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问题 The first electric lamp had two carbon rods from which vapor served to conduct the current across the gap.

选项 A、The first
B、from which
C、served
D、across

答案B应改为by which。

解析 这里需要介词+关系词,按照语意,“在……周围,依赖……手段”用by。
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