Suppose That the Total Fixed Cost for a Guitar Manufacturer

Suppose that the total fixed cost for a guitar manufacturer is $90.The marginal cost of the first guitar produced is $80 and decreases by $10 for the next three guitars produced.Thereafter,the marginal cost increases by $10 for each successive guitar.
-Refer to the information above to answer this question.If the manufacturer can sell her guitars for $100,how many should she produce in order to maximize her profits?

A) 0.
B) 6.
C) 7.
D) 8.
E) 9.