黄土高原农业较发达、人口集中的地区是( )
A:黄土塬
B:黄土川
C:黄土峁
D:黄土梁
7∶28=15∶x
x=____
Molly ____ ____ to come to the airport at six that morning.
若单项式3xm+5y2与﹣5x3y2是同类项,则m的值为____