流感病毒最显著的特点是（）
A.抵抗力强
B.传染性强
C.容易变异
D.免疫性差
E.发病率高
参考答案：C
I ______ her, but I never could.
A．ought to help
B．must help
C．ought to have helped
D．must have helped
梯形螺纹精车刀的背前角应取( )。
A．正值
B．零度
C．负值
