They have been doing their homework

They have been doing their homework They can't have been doing their homework They can’t have been doing their homework ---doing是什么词性?have They can’t have been doing their homework,能不能改成下面句子 They have finished doing their homework(改写句子) They have finished doing their homework.Their homework _ _ _ They can't have been doing thier homework.have been 是现在完成时,但 They are doing their homework?对doing their they don't doing their homework now. they-----[do/doing]their homework 选do还是选doing ( )They have finished their homework,and_______. They must have finished their homework,_______.