-----she was invited to the ball made her very happy A.what B.that C.when D.because

主语从句中that只起引导词作用,不做成分也没有意义she was invited to the ball 成分意义均完整.
而what一般在从句中做主要成分（主语,宾语等）,且有意义（可根据句意灵活翻译）,如：
1.That he came late again made the teacher angry.
2.What he said in class made the teacher angry