根据安全技术措施等级顺序的要求应遵循的原则中，当危险和有害因素存在困难时，可采取（）措施。
参考答案：预防性
— They look worried. What's wrong with them?
— It seems that they something important. [ ]
A．have lost
B．lost
C．had lost
D．lose
下列哪项不是鼓胀病理特点：
A.气滞
B.食积
C.血瘀
D.水停
E.正虚
