每千克苹果a元,买8千克应付______元;当a=2.5时,应付______元.
(1)8a(元),
(2)把a=2.5代入8a得出,8×2.5=20(元),
故答案为:8a;20.
Pleasant music is often played_____classes to make students refreshed and relaxed for a while.
A.between
B.among
C.over
D.after
在同温同压下,任何气体的体积比等于其分子数之比,据此可求得空气中氧气与氮气的分子数之比为____,质量之比为____。