流行病学实验中可比性较好的对照不包括
A．安慰剂对照
B．标准疗法对照
C．历史对照
D．自身对照
E．交叉对照
参考答案：C
When ____ about the source of his income, he hesitated for a moment, obviously not willing to give us a direct answer.
A．asked
B．asking
C．being asked
D．having asked
（）属于完全协和音程。
A.大三度
B.纯四度
C.小六度
D.小七度
