What did you do this morning? (implies it's now afternoon or later) What have you been doing this morning? (implies it's still morning) What have you done this morning? (sounds like you're asking what someone achieved)
The answers would probably be given in the same style as the questions: I went for a bike ride / went cycling / went swimming
I've been swimming and cycling
I've been to the swimming pool and ridden my bike, that's all
I wanted to know what B had done up to the moment I asked him the question, what he had actually achieved that morning which he could boast about. I wanted to practice the present perfect tense. It was only 9.30 am. I did not know that the present perfect continuous could be used too.