一物体做平抛运动,下列四幅图中能正确反映物体水平速度(甲)和竖直速度(乙)随时间变化关系的是( )
A:
B:
C:
D:
It was this exploring around problems and his ____(充满活力的) spirit that led to his most famous invention—the telephone in 1876.
东北地区的城市主要集中在( )
一个由半球和四棱锥组成的几何体,其三视图如图所示.则该几何体的体积为( )