the question is not so easy to understand for a french man like me
I would say A
I think the question is telling us that Q has given this answer to counter the idea of M, what is M hypthesis ? I think it's A
M was going to say that people using right hand live longer than the people using the left hand, but Q explained that before children had to use the right hand, which explains that you can not make the difference between people who were using their right hand and those who were using their left hand...
Maybe I misunderstood the question.
A for me