Production Costs

78. If the total cost of producing 10 units is $100 and the marginal cost of the eleventh unit is $21, then which of the following is NECESSARILY true?
A) Total variable costs of 11 units are $121.
B) Total fixed costs are $79.
C) The marginal cost of the tenth unit is more than $21.
D) The average total cost of 11 units is $11.
E) The average total cost of 12 units is $12