It is raining heavily, none of the students are late for school.
A.although B.but C.so D.because
B
【解析】
句意:尽管在下大雨,但是没有一个学生上学迟到。A. although 尽管; B. but 但是; C. so因此; D. because因为;根据It is raining heavily,和none of the students are late for school.之间是转折关系;故选B