Choose the method of pay that would result in the most earnings for one month on sales of $40,000.

a.) straight commission of 7.5% on all sales.
b.) monthly salary of $1,600 plus 2.5% commission on all sales.
c.) graduated commission of 5% on the first $30,000 in sales and 6% on anything over that.
d.) graduated commission of 4% on the first $25,000 in sales and 8% on anything over that.