Which of These Are Some Problems Encountered in Long-Term Studies

Which of these are some problems encountered in long-term studies of the effects of fitness training in children?

A) Because of the wide variety in children's bodies, training effects are unlikely.
B) Difficulty in getting children's cooperation over long periods of time
C) Getting children to participate in uncomfortable, high intensity training
D) All of these are correct.
E) Difficulty in getting children's cooperation over long periods of time and getting children to participate in uncomfortable, high intensity training only