Mark's Mother Is Worried Because He Only Sleeps About Four

Mark's mother is worried because he only sleeps about four to five hours per night. What would you tell her?

A) There is probably nothing to be worried about if Mark is healthy and productive since different people need differing amounts of sleep.
B) There is no problem, unless he spends most of his sleep time in REM sleep.
C) She has reason to be concerned because, in the long run, Mark's lack of sleep will take its toll physically.
D) There is nothing to worry about because young people need less sleep than older people.