问题
选择题
－He asked Tom, “Have you finished your homework？”
－He asked Tom .
A．if had he finished his homework.
B．whether he had finished his homework
C．if he had finished your homework.
D．if you had finished your homework.
答案
答案：B
连词辨析。If和weather都能表示是否的意思，但是if还可以引导条件从句，表示如果。当可能引起歧义时就只能用weather了。宾语从句中一般为陈述语序，可排除AD选项。选项C还可以表达，如果他完成了作业，他问汤姆的意思。故选B最符合上文语义。
和
的两个相邻的条形区域中分别有匀强磁场和匀强电场，磁场方向垂直于纸面向里，电场方向与电、磁场分界线平行向右。一带正电的粒子以速率v从磁场区域的上边界的P点向下成θ=60º射入磁场，然后以垂直于电、磁场分界线的方向进入电场，最后从电场边界上的Q点射出。已知PQ垂直于电场方向，不计重力。求电场强度和磁感应强度大小之比，以及粒子在磁场与电场中运动的时间之比。