题目

The students were singing an English song _______ Mr. Xiang came into the classroom.A. after B. when C. before D. as 答案：【答案】B【解析】句意：项先生走进教室时，学生正在唱一首英文歌曲。考查连词辨析。A. after在…之后；B. when当….的时候，从句的动作可以与主句的动作同时发生，也可以先于主句的动作发生，由when引导的时间状语从句，主句用过去进行时，从句应用一般过去时；例如，It was snowing when he arrived at the已知aR,问复数z=（a2－2a+4）－（a2－2a+2）i所对应的点在第几象限?复数z对应点的轨迹是什么?