What are you doing? 你在干什么？

1.What are you doing? 你在干什么？ 
2. I’m studying my lesson. 我在做我的功课
3. How old are you? 你多大了? 
4. I’m twenty-one years old. 我21岁了。 
5. I’m two years older than you are. 我比你大两岁。