Past Perfect Continuous em exercício

Could somebody help me with this question?

Put the verb into the most appropriate form, past continuous (I was doing), past perfect (I had done), or past perfect continuous (I had been doing)

1. When I arrived, everybody was sitting arround the table and talking. Their mouths were empty, but their stomachs were full. They...(eat).

I answered "They had eaten", but the right answer is "They had been eating. I would like to know why. I think that is because the act of eat was recently in the sentence, but I'm not sure. Thank you all for the supporting.