In perfect​ competition, a​ firm's marginal revenue is A.
equal to price.
B.
the change in the​ firm's total revenue divided by the change in the​ firm's output.
C.
the additional revenue the firm earns when it sells one more unit of output.
D.
All of the above.
E.
Only A and B above.

D.  All of the above is the correct answer.

Explanation:

d.

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answer; no longer than a reasonable period of time;