填空题 (The first) electric lamp had two carbon rods (from which) vapor (served) to conduct the current (across) the gap.
A. The first B. from which C. served D. across

  • 1、
【正确答案】 1、B应改为by which。    
【答案解析】[解析] 这里需要介词+关系词，按照语意，“在……周围，依赖……手段”用by。