Mr. Hall understands that ______ maths has always been easy for

Mr. Hall understands that ______ maths has always been easy for him, it is not easy for the students.
A、unless
B、since
C、although
D、when
【正确答案】：C
【题目解析】：该题考查连词在宾语从句中的用法.观察题干Mr. Hall understands是主句,其后有that引导的宾语从句,可将该句简化为: ______ maths has always been easy for him, it is not easy for the students.分析简化后的句子可知, maths has always been easy for him与it is not easy for the students之间含有转折关系，其他选项不合题干逻辑。故答案为C。