You may have noticed that the outcomes of some variables are restricted to two outcomes such as male-female, clean-dirty, and broken-not broken etc. If you carefully study the environment we live in, you may see that sometimes populations consist of only two categories/classes or two subpopulations. For example, as we said before, such classes are male and female, dead and alive, married and not married. The population proportions of two categories/classes are denoted by, in general, P and Q. 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. The correct answer is A.