At Bert's Bootery,the Total Cost of Producing Twenty Pairs of Boots

At Bert's Bootery,the total cost of producing twenty pairs of boots is $400.The marginal cost of producing the twenty-first pair of boots is $83.We can conclude that the average

A) variable cost of 21 pairs of boots is $23.
B) total cost of 21 pairs of boots is $23.
C) total cost of 21 pairs of boots is $15.09.
D) total cost of 21 pairs of boots cannot be calculated from the information given.