正确答案：C

　　答案解析：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。