超声波生物效应主要指（）
A.振动和传输热效应
B.吸收和反射
C.机械效应、热效应和空化效应
D.散射与非线性效应
E.多普勒效应
参考答案：C
“一人之下，万人之上”的丞相是皇帝的重要辅臣，而废除这一官职的是哪一朝代（ ）
A．隋朝
B．唐朝
C．明朝
D．清朝
—Why didn't Tom come to school today?
—Sorry, I don't know______ come and later I'll give him a call. [ ]
A. why didn't he
B. why he didn't
C. why did he
D. why he
