They can’t have been doing their homework

They can’t have been doing their homework--句中“can't"是不是可以直接去掉，从语法上来讲没有错啊？？？
还有They can’t have been doing their homework能改成：
They haven't been doing their homework 吗？？？

They can't have been watching the football match this time yesterday,___? They can can what they can can,but can't can what they can't can是什么意思 can't have been They must have arrived by now,haven't they? they are [ ] children that they can't dress themselves. they can help peopleto w_____out many problems they can't do easily. They say you can’t judge a book by They can't understand what the foreigners say,they can't read English books and newspapers. They didn't do as well as they should have can I have ____water?No you can't have ____.