Nobody except Tom and Millie 　　　 that park before. Only they can be our guides.

A．have been to

B．has been to

C．has gone to

D．have gone to

按要求画图。

（1）画出小球的受力情况；

（2）木块与斜面粗糙，木块沿斜面匀速下滑，画出木块的受力情况。