Mary handed in her exam paper, she then realized that she had missed one question.

A. After Mary realized that she had missed one question, she handed in her exam paper.

B. Not until Mary handed in her exam paper did she realize that she had missed one question.

C. Before Mary handed in her exam paper, she realized that she had missed one question.

D. Only after Mary realized that she had missed one question did she hand in her exam paper.