______ of that country _______ by snow.
A:Second thirds; is covered
B:Two thirds; is covered
C:Two thirds; are covered
D:Two third; covers
表层土的养分来自腐烂的动植物的遗体,它们被称为____。
现代城市,____、____已经成为制约城市发展日益突出的问题。
如图,四边形PAOB是扇形OMN的内接矩形,顶点P在弧MN上,且不与M,N重合,当P点在弧MN上移动时,矩形PAOB的形状、大小随之变化,则PA2+PB2的值( )