Keith Is 5 Years Old and Sucks His Thumb Excessively

Keith is 5 years old and sucks his thumb excessively. Keith's mother should

A) allow Keith to suck his thumb because his primary teeth are unaffected by this habit.
B) discourage Keith from sucking his thumb and give him a pacifier instead.
C) allow Keith to suck his thumb because this helps to calm him down when he gets difficult to manage.
D) discourage him from sucking his thumb because this habit is likely to damage his permanent teeth.