A doctor has told a patient to take vitamin pills. The patient needs at least 21 units of vitamin A, at least 8 units of vitamin C, and at least 71 units of vitamin D. The red vitamin pills cost 20 cents each and contain 7 units of A, 1 unit of C, and 7 units of D. The blue vitamin pills cost 40 cents each and contain 2 units of A, 1 units of C, and 10 units of D. How many pills should the patient take each day to minimize the cost?
a.8 red and 0 blue
b. 4 red and 4 blue
c. 5 red and 3 blue
d. 3 red and 5 blue
