湍流的产生和强弱与（）因子有关。
A.物理
B.热力和动力
C.重力
D.下沉气流
参考答案：B
You look unhappy. What ________ just now? [ ]
A. did you happen
B. did happen to you
C. happened to you
D. you happened
邀请招标投标阶段是指( )。
A．从发布招标广告始到投标截止日期为止的期间
B．从发出邀请函始到投标截止日期为止的期间
C．从发出邀请函到签定合同日期为止的期间
D．从发出邀请函始到定标为止的期间
