102年台水評價人員 英文 - 考試

出處: 102年台水英文考題

He started his own business five years ago. That was ________ he began to be
independent of his parents.

1. where 2. when 3.what 4.which

我選3 答案為2

我的想法是說WAS之後沒有先行詞,所以應該要用WHAT才對吧?

WHEN是關係副詞是介係詞+關代,可是這題並沒有先行詞阿...

請問如何解這題呢? 有沒有高手能幫忙?

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