______of the U.S. Senate stands for re－election every two years.A．4／5B．3／4C．2／3D．1／3

______of the U.S. Senate stands for re-election every two years.

A．4/5

B．3/4

C．2/3

D．1/3

请帮忙给出正确答案和分析，谢谢！