性身份障碍包括（） A.易性癖 B.恋物癖 C.同性恋 D.异装癖 E.恋童癖

下列能判定一个四边形为平行四边形的条件是  [ ]

A.一组对边平行，另一组对边相等    

B.一组对边平行，一组对角互补    

C.一组对角相等，一组邻角互补      

D.一组对角相等，另一组对角互补

Tom along with his classmates ______ for the coming exam.

A．are preparing

B．are preparing themselves

C．is preparing him

D．is preparing himself

—The problem is very difficult. I'm afraid you can't work it out now.

   —     I fail this time, I will try it again.

A．Even if

B．Since

C．As if

D．Though

如下图所示电路，换路前I_L(0-)=0，则该电路换路后的电压U_L(t)等于（）(设 )。

A．A

B．B

C．C

D．D

落在某一特定类别或组中的数据个数称为（）。

A.频数

B.频率

C.频数分布表

D.累积频数