You may not replace 'what' with 'who'.Code:When the helicopter took off with the thing which (=what) was to be the last survivor, he watched everything in the world move away from him, and he let that happen.
If you have difficulty understanding the meaning of the relative pronoun, what, you could replace it with 'the thing which'.
In the sentence in question, you may use 'the person who' in place of 'what'.
I hope it helps you.
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