people don't know the value ___they have until it's gone.A that B what C which很抱歉，此题错了，不必回答了。

people don't know the value ___they have until it's gone.A that B what C which很抱歉，此题错了，不必回答了。
people don't know the value ___they have until it's gone.A that B what C which
很抱歉，此题错了，不必回答了。

people don't know the value ___they have until it's gone.A that B what C which很抱歉，此题错了，不必回答了。
A that
岂不在作弄人吗?