Mark the letter A, B, C, or D to indicate the sentence that best combines each pair of sentences in the following questions.
The teacher was giving the lesson. The lights went out.
A. The lights went out as soon as the teacher started giving the lesson.
B. If the teacher had been giving the lesson, the lights wouldn't have gone out.
C. The teacher was giving the lesson, so the lights went out.
D. While the teacher was giving the lesson, the lights went out.