据唐代天宝年间统计的全国户口数字,秦岭、淮河以南地区的户数占45.5%,以北占到54.5%;据北宋《太平寰宇记》所载北宋初期全国户数,秦岭、淮河以南地区的户数占59.1%,以北则为40.9%。这一变化( )
— Peter doesn't mind lending you his camera.
— He ________. I've already borrowed one.
如图所示,用细线吊着一个质量为m的小球,使小球在水平面内做圆锥摆运动,关于小球受力,正确的是( )