Which Situation Is an Example of Modifier Gene Action

Multiple Choice

Which situation is an example of modifier gene action?

A) A mother and father both have type O blood, and their son is born with type B blood.
B) Two children in the same family both have sickle cell disease, but one child has more severe symptoms than the other.
C) Monozygotic twins separated at birth and raised in two different countries are of different heights (3 inches) and different weights (25 pounds) .
D) A woman who has a gene mutation for diabetes mellitus type 2 delays the onset of the disease by exercising and maintaining a normal weight.