机体再次接触相应抗原一般是24小时后即发生接触性皮炎,多少小时达高峰()
A.24~48
B.24~72
C.48~72
D.48~96
E.72~96
参考答案:D
在光滑水平面上有一木块保持静止,子弹穿过木块,下列说法中正确的是 [ ]
A.摩擦力做功使木块内能改变
B.子弹损失的机械能,等于子弹与木块增加的内能
C.子弹损失的机械能,等于木块动能的增加和木块、子弹内能增加的总和
D.子弹与木块总动能守恒
There is no doubt _____, in my opinion, _____ matters is not the speed, but the quality of the product. [ ]
A. what; what
B. that; that
C. that; if
D. that; what