On a typical day, ABCD corporation writes 8,000 in checks. It generally takes 3 days for those checks to clear. Each day the firm typically receives $13,000 in checks that take 4 days to clear. What is the firm's average net float? (Please show work)
A- 5,000
B- 21,000
C- 28,000
D- -28,000
E- -5,000