乙型血友病缺乏的因子为（）
A.因子Ⅺ
B.因子Ⅸ
C.因子ⅩⅡ
D.因子Ⅷ
E.因子Ⅹ
参考答案：B
--- Does Susan like singing?
--- Yes, she does. But she prefers __________ to _________.
A．to dance; sing
B．to dance; singing
C．dancing; sing
D．dancing; singing
光纤传播采用什么原理（）
A、全反射
B、折射
C、全反射和折射
D、反射原理
