When the price faced by a competitive firm was $5, the firm produced nothing in the short run. However, when the price rose to $10, the firm produced 100 tons of output. From this we can infer that:_______

a. the firm's marginal cost curve must be flat.
b. the firm's marginal costs of production never fall below $5.
c. the firm's average cost of production was less than $10.
d. the firm's total cost of producing 100 tons is less than $1000.
e. the minimum value of the firm's average variable cost lies between $5 and $10.