强调句判断的依据就是去掉It was ...that之后句意不变，语法通顺。

It was in 1949 that China was founded. In 1949 China was founded. 你就记住把it was和that去掉看句子语法对不对 完不完整就可以了。

如果填which和when从句应该是定语从句 修饰1949这个年份 It was 1949 when China was founded. It was 1949 in which China was founded. 题目上的句子中in 1949这个介词短语在句子中做状语 用it is…that强调句就是强调这个状语 而把in去掉只剩1949的话整个句子不完整了。