生物不安全性按污染的性质可分为（）不安全因素。
A.生物性
B.化学性
C.物理性
D.非生物性
参考答案：A, B, C
什么样的场合下可以使用军旗？
His ability has never been in doubt----the question is ___________ or not he is prepared to work hard.
A．that
B．if
C．where
D．whether
