(对划线部分提问)David and Linda have been watching TV for four hours.(watching TV划线)

问题描述:

(对划线部分提问)David and Linda have been watching TV for four hours.(watching TV划线)
David and Linda have been watching TV for four hours.(watching TV划线)
_____ _____ David and Linda _____ _____ for four hours?

What have ……been doing