药材浸提过程中渗透和扩散的推动力为（）
A.被动扩散
B.浓度差
C.主动转运
D.胞饮
E.温度差
参考答案：B
—How often do you take exercise?
—_____ [ ]
A. Sometimes
B. Three hours
C. At three o'clock
D. In two hours
风电场必须建立哪些技术档案？
