问题补充：
Mr.Hall understands thatmaths has always been easy for him，it is not easy for the students.
A. unless
B. since
C. although
D. when
答案：
A. unless
B. since
(答案→)C. although
D. when
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。
