A retailer spends $500 per month to keep its online shop active and updated.
The store acquires shirts at a marginal cost of $5 per shirt. Each shirt sells
for a marginal benefit of $10 per shirt.
What is the minimum number of shirts the retailer needs to sell in order to
pay for all its costs in a month?
A. 100
B. 125
C. 25
D. 75