A researcher wants to compare the effect of a new type of shampoo on hair condition. The researcher believes that men and women may react to the shampoo differently. Additionally, the researcher believes that the shampoo will react differently on hair that is dyed. The subjects are split into four groups: men who dye their hair; men who do not dye their hair; women who dye their hair; women who do not dye their hair. Subjects in each group are randomly assigned to the new shampoo and the old shampoo. This experiment

a. has three factors (shampoo type, gender, whether hair is dyed).
b. has two factors (shampoo type and whether hair is dyed) blocked by gender
c. has one factor (shampoo type), blocked by gender and whether hair is dyed.
d. has two factors (gender and whether hair is dyed) blocked by shampoo type.
e. is completely randomized. (Old exam question)