请问这句英语中的in which代指什么?
问题描述：
请问这句英语中的in which代指什么?
dementia is a serious illness affecting someone's brain and memory in which they gradually stop being able to think or behave in a normal way.Most forms of dementia usually affect only old people.
答
in which 指代a serious illness ,简单说,先行词a serious illness 在这里是一种抽象地点名词.