己烯雌酚遇光易变质是因为分子中含有
A．苯环
B．烯键
C．羟基
D．酚羟基
E．乙基
参考答案：D
He _____ without saying good-bye to them, for he always has good manners.
A．mustn’t have left
B．may not leave
C．shouldn’t have left
D．can’t have left
电力变压器的检修，分（）、（）和（）。
He _____ without saying good-bye to them, for he always has good manners.