Answer:
their capacity for listening has been reached.
Step-by-step explanation:
According to the working memory theory of listening, when the listening capacity of the human being is finally reached, people find it hard to remember what they hears.
It is theory where it becomes difficult to concentrate and remember what they hears when people's short term memory is reached or is full. It occurs when people have already heard many things and there is no practical no free space for new information to be remembered.
Hence the answer is ---
"their capacity for listening has been reached".