Kens Shirt Shop sells custom shirts for $50. The shops sales revenue is $4,000, the cost of goods sold is $1,500, total fixed expenses are $950, and the variable cost per shirt is $28. Which of the following would Ken use to calculate his contribution margin per shirt?

A : $4, 000 $1,500

B : $50 $28

C : $4, 000 $28 $950

D : $1,500 $28 $50