Which of the Following Calculations Requires the Use of the Addition

Which of the following calculations requires the use of the addition rule of probability?

A) Calculating the probability of green plants from the cross AaGg × AaGg, where G is the symbol for black and A is the symbol for tall.
B) Calculating the probability that parents who are heterozygous for both cystic fibrosis and polydactyly genes will have a child with both cystic fibrosis and polydactyly.
C) Calculating the probability that a cross between an Hh and an hh plant will result in four offspring that are each homozygous recessive.
D) Calculating the probability of a child having either sickle-cell anemia or cystic fibrosis if parents are each heterozygous for both.