Mr Hall understands that ________ maths has always been easy for him,it is not easy for the studants

Mr Hall understands that ________ maths has always been easy for him,it is not easy for the studants．

A.unless

B.since

C.although

D.when

答案及解析： C

解析：

解析：根据前后题意推断出前后两个分句意义是相反的，只有C项符合，解题关键在于把握语境。