I'm sure that they were listening to the radio. they ____ ____been listening to the radio.同义句

I'm sure that they were listening to the radio. they ____ ____been listening to the radio.同义句
I'm sure that they were listening to the radio. they ____ ____been listening to the radio.同义句

I'm sure that they were listening to the radio. they ____ ____been listening to the radio.同义句
更正楼上答案如下：
= They (must have) been listening to the raido.
must have been doing sth （当时）一定在做某事
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must be
must be doing，一定在做某事

must have