当某种因素与疾病无关联时（）
A.O=R-O
B.OR>0
C.OR<1
D.OR>1
E.OR=1
参考答案：E
33. He waited until it was dark ____ nobody could see him clearly.
A．so that
B．in order to
C．although
D．However
检查多级泵的轴瓦接触应占总面积的（）%以上。
A、50
B、60
C、70
D、80
