高频方向保护是利用高频信号比较被保护线路两端的（）。
A.功率方向
B.电流相位
C.电压大小
D.功率大小
参考答案：A
Tom realized that he ________ his physics book at home when he reached school．
A．left
B．had left
C．was leaving
D．leaves
_______ Changjiang River is one of ______ longest rivers in ______ world.
A．The; the; the
B．The; a; a
C．A; the; a
D．/; the; the
