The thought of going back home was all that kept him happy while he was working abrosd.
【答】如果 all 是先行词，定语从句的关系代词要用 that ,不能用 which。这里的all 是先行词，that kept him happy 是定语从句。while he was working abrosd 是时间状语从句，

有些不定代词如果做先行词，后面的关系代词只能用that,all是不定代词。

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The thought of going back home was all that kept him happy while he was working abroad
【问】句中为什么要用which 不能用 that ？
【答】如果 all 是定语从句修饰的先行词，定语从句的关系代词要用 that ,不能用 which。
这里的all 是先行词，that kept him happy 是定语从句，所以不能用which ， 应该用 .that。while he was working abroad 是时间状语从句。