Frank's father, _____works in a foreign trade company, always goes on a business trip.
A. that
B. who
C. whom
D. as
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He might have been taken into prison now if he (join)_____the kidnappers.
During the holidays, the concert enjoys huge(popular)______despite the high ticket price.
Arrange the following events in correct order.
已知一高斯面所包围的体积内自由电荷代数和为零，则（ ）
A. 高斯面上各点的场强均为零
B. 高斯面上各点的电位移均为零
C. 穿过整个高斯面的电位移的通量为零
D. 以上说法都不对
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