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.
The teacher has done his best to help all students. However, none of them made any effort on their part.
A. The teacher has done his best to help all students, then, none of them made any effort on their part
B. Although the teacher has done his best to help all students, none of them made any effort on their part
C. Because the teacher has done his best to help all students, none of them made any effort on their part
D. If the teacher has done his best to help all students, none of them made any effort on their part