In two common species of flowers, A and B, the proportions of flowers that are blue are papa and pbpb , respectively. Suppose that independent random samples of 50 species-A flowers and 100 species-B flowers are selected. Let pˆap^a be the sample proportion of blue species-A flowers and pˆbp^b be the sample proportion of blue species-B flowers. What is the mean of the sampling distribution of pˆa−pˆbp^a−p^b ?