立井中升降人员或升降人员和物料的提升装置，卷筒上缠绕的钢丝绳层数不准超过（）层。
A.1
B.2
C.3
D.4
参考答案：A
________the problem, he wouldn't have committed those mistakes.
A．If he understood
B．Had he understood
C．When he had understood
D．If he would understood
清热宁嗽化痰定喘丸功效与作用
