this is the best film that i have ever seen为什么不用which

this is the best film that i have ever seen为什么不用which
this is the best film that i have ever seen
为什么不用which

this is the best film that i have ever seen为什么不用which
先行词有形容词最高级时只能用THAT ,不能用WHICH
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先行词 film 前有最高级 best ，所以只能用that，不能用which 。

祝你开心。