According to the endosymbiotic hypothesis, why did large cells not destroy the small aerobic cells they engulfed

I was waiting to get some choices to choose the correct answer. I am trying to answer this question from research and hope that it helps you. According to the endosymbiotic hypothesis, the large cells did not destroy the small aerobic cells they engulfed because they were a source of energy. These small aerobic cells were beneficial.

The answer is observations and interpreting.