A Manufacturer Sells Watches for $80 Per Unit

Multiple Choice

A manufacturer sells watches for $80 per unit. The fixed costs related to this product are $8,000 per month, and the variable costs are $60 per unit. This gives a profit function , where r is the revenue per unit and c is the variable cost per unit. If the manufacturer has a profit of zero, how many watches were sold? ​

A) 800
B) 4,000
C) 400
D) 100
E) 133