(a) $8,000 is the upfront cost, also known as the fixed cost. This expense is incurred regardless of how many items are produced because even when q = 0, the cost is still 8,000.
(b) $12 is the marginal cost, also known as the variable cost. This is how much additional cost is incurred from producing each additional item. It is like the slope m in y = mx + b. In this cost function, every 1 additional item produced incurs an additional $12 in expense. It can also be viewed as the rate of change of cost relative to the quantity of items, and it is equivalent to the derivative of C(q) or C'(q).
