将下列句子改写为现在完成时态。

1. She went to Beijing yesterday. (already)        

    She _____ _____ to Beijing already.    

2. Did he do anything today? (yet)         

    _____ he _____ anything _____?    

3. We saw each other two days ago. (since)        

    We _____ ____ each other two days _____.   

4. What is he doing now? (these days)        

    What _____ he _____ _____ these days?

关于力的认识，下列说法中正确的是（　　）

A．国旗被宇航员带到太空后重力不变

B．物体间接触面积变大时，滑动摩擦力变大

C．只有相互接触的物体才会产生力的作用

D．物体间力的作用是相互的