-If 6 Workers Can Wash 42 Cars a Day and 7

-If 6 workers can wash 42 cars a day and 7 workers can wash 50 cars a day, then the marginal product of the 7th worker equals

A) 50 cars a day.
B) 7 cars a day.
C) 42 cars a day.
D) 7.1 cars a day.
E) 8 cars a day.