They are 【typing the letter】.用括号里的提问
问题描述：
They are 【typing the letter】.用括号里的提问
答
What are they doing?
针对动词词组提问，用what ，句中有 are， are提前放到what后边，然后看画线的词组是动词ing形式，就用doing，是动词原型就用 do
答
提问：
What are they doing？
答
What are they doing?
答
what are they doing