A:3645粒
B:1215粒
C:2430粒
D:4860粒
—_______ is your pet dog?
—It's ten months old.
you sing?
一个球的质量为m,用绳系着放置在倾角为θ的光滑斜面上,细绳处于竖直方向上,如图所示,则斜面对球的支持力为( )