Mark the letter A, B, C, or D on your answer sheet to indicate the sentence that best combines each pair of sentences in the following questions.
He is late again. I don't know the reason why.
A. I don't know the reason why he is late again.
B. Why he is late again is I don't know the reason.
C. Why I don't know the reason he is late.
D. The reason why he is late again I don't know.