因为这里can't表猜测或判断,不可能的意思,所以反义疑问词应该对问题本身提问,而不是对判断词提问.所以AB是错的.
为什么C是错的?
首先我们来把原句恢复成陈述句:
They can't have been watching the football match this time yesterday.
注意,这里的can't表示不可能.
再变成肯定句：
They were watching the football match this time yesterday.
为什么can't have been watching对应were watching?
原因是：因为是this time yesterday,所以是过去时态,如果变成can't be watching就变成现在时态了.为了表现推测的内容是过去,所以要用can't have been doing,但这里绝对不是完成时态,所以C是错误的.