Suppose that a new alloy is invented which uses copper and zinc in fixed proportions, where 1 unit of output requires 3 units of copper and 3 units of zinc for each unit of alloy produced. If no other inputs are needed, the price of copper is 2, and the price of zinc is 2, what is the average cost per unit when 4,000 units of the alloy are produced?

a) 6.33

b) 666.67

c) 0.67

d) 12

e) 6,333.33