Olga handed in her exam paper. She then realised that she had missed one question.

A. Having realised that she had missed one question, Olga handed in her exam paper.

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

C. Had Olga realised that she had missed one question, she wouldn't hand in her exam paper.

D. Only after Olga realised that she had missed one Question did she hand in her exam paper.