如图所示皮带轮,A轮是主动轮,B轮是从动轮.则传送带上的P点、从动轮上的Q点所受摩擦力方向判断正确的是( )
A:P点向下,Q点向上
B:P点向上,Q点向上
C:P点向上,Q点向下
D:P点向下,Q点向下
-- Judy's never been late for school, she?
-- . She is always the first to get to school every morning.
贵州省的主要矿产种类是( )
如果单项式﹣xa+1y3与x2yb是同类项,那么a、b的值分别为( )