3) 有少数动词用两种时态都可以，意思差别不大，如：
They have been working very well this term. 这学期他们学习很好。
They have worked very well this term.

It has been raining for two hours. 下了两个小时的雨。
It has rained for two hours.

I have been hearing from her regularly. 我定期收到她的来信。
I have heard from her regularly.

We have looked for mushrooms but have not found any. 我们找寻蘑菇但一无所获。