填空题 {{U}}The first{{/U}} electric lamp had two carbon rods {{U}}from which{{/U}} vapor {{U}}served{{/U}} to conduct the current {{U}}across{{/U}} the gap. A. The first B. from which C. served D. across
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【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。