Assume That Marginal Revenue Equals Rising Marginal Cost at 100

Assume that marginal revenue equals rising marginal cost at 100 units of output. At this output level, a profit-maximizing firm's total fixed cost is $600 and its total variable cost is $400. If the price of the product is $8 per unit and the firm produces the profit-maximizing level of output, the firm will earn an economic profit of

A) -$200.
B) zero.
C) $100.
D) $200.
E) $800.