The firm's short-run total cost function is C(Q) = 70 + 4Q + 2Q^2, where Q is the amount of output. The corresponding unit cost functions are:
A. AFC = 70, AVC = 4Q + 2Q^2, MC = 4Q
B. AFC = 70, AVC = 4 + 2Q, MC = 4 + 4Q
C. AFC = 70/Q, AVC = 4 + 2Q, MC = 4 + 4Q
D. AFC = 70/Q, AVC = 4 + 2Q, MC = 4 + 2Q