6. Color blindness is an X-linked recessive hereditary disease due to the deficiency of specific photoreceptors in the retina. One man with color blindness got married to a carrier woman. If they give birth to two boys, what is the probability of both offspring being afflicted with this disease? (A) 1/2 (B) 1/4 (C) 1/8 (D) 1/16 (E) 1/64