During the class, Tom slipped out of the classroom ____the back door so as not to be found by the

teacher.[ ]

A. across

B. in

C. by

D. beside

A firm operating under perfect competition will experience economic losses when which of the following conditions exists（）

A. Marginal revenue is greater than average total cost.

B. Market price is less than average total cost.

C. Marginal cost = average total cost = marginal revenue = price.