加拿大是这一时期唯一服刑人数在减少的（国家）
Canada主语
is谓语
the only 表语指代加拿大
in which the numbers in prison have decreased 定语从句修饰the only,which指代一个国家在从句中作状语,前加in
over the period.时间状语我不太明白为什么要必须这么说Canada is the only which the numbers in prison have decreased over the period . 可以吗，不知道为什么加IN定语从句变成单句为the numbers in prison have decreased in Canada in Canada 做地点状语Canada 作介词in的宾语，化为定语从句Canada 用which代替，一般介词一并前置做关联词组，in which 也可用where替换