In two common species of flowers, a and b, the proportions of flowers that are blue are pa and pb , respectively. Suppose that independent random samples of 50 species-a flowers and 100 species-b flowers are selected. Let pˆa be the sample proportion of blue species-a flowers and pˆb be the sample proportion of blue species-b flowers. What is the mean of the sampling distribution of pˆa−pˆb ?.