To investigate whether there is a difference in opinion on a certain proposal between two voting districts, a and b, two independent random samples were taken. From district a, 35 of the 50 voters selected were in favor of the proposal, and from district b, 36 of the 60 voters selected were in favor of the proposal. Which of the following is the test statistic for the appropriate test to investigate whether there is a difference in the proportion of voters who are in favor of the proposal between the two districts (district a minus district b)?.