一般回答用 I am doing sth.
What are you doing? 是用来询问你现在正在干什么，是现在进行时的一种表现形式，表示现在进行的动作或存在的状态。
例如：What are you doing?
I am watching TV./ I am reading a book./I am eating an egg
使用现在进行时的情况
1、描述某个动作目前正在发生：You are using the Internet. You are studying English grammar.
2、描述某个动作正在这段时间或趋势发生：Are you still working for the same company? More and more people are becoming vegetarian.
3、描述某个未来的动作或事件，而这个动作或事件已经是规划或准备好的：We're going on holiday tomorrow. I'm meeting my boyfriend tonight. Are they visiting you next winter?
4、描述一个暂时的事件或情况：He usually plays the drums, but he's playing bass guitar tonight. The weather forecast was good, but it's raining at the moment.
5、用 always, forever, constantly 等副词来描述和强调一个不断反覆发生的动作：Harry and Sallyare always arguing!You're constantly complainingabout your mother-in-law!