What is the difference between marginal cost and marginal revenue?

A) Marginal cost is the money earned from selling one more unit of a good. Marginal revenue is the money paid for producing one more unit of a good.
B) Marginal cost is the money paid for producing one more unit of a good. Marginal revenue is the money earned from selling one more unit of a good.
C) Marginal cost is the money a producer might make from one more unit. Marginal revenue is the money a producer actually makes from one more unit.
D) Marginal cost is the money a producer actually makes from one more unit. Marginal revenue is the money a producer might make from one more unit.