英语试题He was nearly drowned once.-When was ( -( ) was in 1998

英语试题He was nearly drowned once.-When was ( -( ) was in 1998 ( ) he was in middle school.
-He was nearly drowned once.
-When was (
-( ) was in 1998 ( ) he was in middle school.
我知道答案是that it when
但是为什么最后一个when啊?这不是强调句吗?那应该是that啊?
于是我看到一种解释是:答句属于变相的强调句,当表示时间的时候,强调句中的that可以换成when,所以去掉it was...when...再调整一下语序,就是 He was in middle school in 1998 .
但是我看到另一种解释是:when引导的是定语从句.如果加逗号,就是非限制性定语从句.时间状语从句是用来修饰句子的谓语动词的,表示动作发生的时间.例如：
It happened when I was living in the country.
这事发生在我住在农村的时候.