Neither of Sean's Parents Has a Red-Green Color Deficiency, but Sean

Neither of Sean's parents has a red-green color deficiency, but Sean does. Why is this possible?

A) Both of Sean's parents must be heterozygous for the gene for red-green color deficiency.
B) Both of Sean's parents must be homozygous for the dominant allele for red-green color vision.
C) Sean's father must be heterozygous for the gene for red-green color deficiency.
D) Sean's mother must be heterozygous for the gene for red-green color deficiency.