Pricing

ABC Company is about to launch a new product. Facilities will allow the company to produce up to 20 units per week.
The marketing department has estimated that at a price of $8,000 no units will be sold, but for each $150 reduction in price one additional unit per week will be sold.
Fixed costs associated with manufacture are expected to be $12,000 per week.

Variable costs are expected to be $4,000 per unit for each of the first 10 units; thereafter each unit will cost $400 more than the preceding one.

The most profitable level of output per week for the new product is:
A 10 units
B 11 units
C 13 units
D 20 units

In P= a-bQ
a will be 8000
b will be 150

MC = 400

I am getting the answer of 25 units.
Can you please explain me the marginal cost to be used in this question to derive the optimal quantity? I am stuck!!!