If we know the proportion P for one class, the proportion of Q for the other class must be equal to 1 – P. Because the total probability must be equal to 1 and there are only two possible outcomes. For example, if the proportion of married people in a group is P = 0.63 then the proportion of the single in that group is Q=1-P = 1- 0.63 = 0.37. The correct answer is C.