As a simple and convenient way to get ______ latest fashions online, online shopping also has ______ major problem — you can’t see whether they fit or not.

A．the; the

B．/ ; the

C．the ; a

D．/; /

要改变物体原来的运动状态，使静止的物体运动，使运动着的物体改变方向和速度，就必须对物体施加外力。（）