老年人跌倒常见危险因素中属外在因素的是（）
A.生理因素
B.心理因素
C.疾病因素
D.药物因素
E.环境中的危险因素
参考答案：E
The boy was so tired that he fell asleep _______ his mother could take off his clothes.
A．before
B．after
C．while
D．when
组织土石坝综合机械化施工的基本原则是什么？
