Mark's Mother Is Worried Because He Sleeps Only Six or Seven

Mark's mother is worried because he sleeps only six or seven hours per night,and she believes that people need at least eight hours of sleep.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.