Two parents are phenotypically normal, but one of their four biological children has a typical autosomal recessive trait. The other three children are phenotypically normal. It is very likely that:

A) the affected child is a girl.
B) the affected child is a boy.
C) the trait was expressed by one of the grandparents of the children.
D) the parents are both heterozygous for the trait.
E) if the affected child eventually marries a phenotypically normal spouse, all of their children will have the trait.