Take a look at this answer to a similar question which explains the difference in detail -
Hope it helps!
Don't confuse the interrogative pronoun qui/que (who/what) with the relative pronoun qui/que. The relative pronoun is qui when it functions as the subject of the clause and que when it's the object. That is regardless of whether it's an object or a person.
Qu'est-ce qui a changé? -- (lit.) What is it that changed?
This example sentence actually features both versions of qui/que. At the beginning it is que (what) and in the relative clause it is qui because it stands in for the subject of the clause.
But why not “ Qu’ est-ce qu’ a changé ? Rather than “Qu’ est-ce qui a changé” ? I don’t think the instruction page gave this example or explained it, but it was tested.
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