Difference between why and what?

[Isram]

English Grammar Tests, Elementary Level

ESL/EFL Test #47 "Question Words", question 8

This is ......... I want to be when I grow up.

(a) why
(b) when
(c) which
(d) what

English Grammar Tests, Elementary Level

ESL/EFL Test #47 "Question Words", answer 8

This is what I want to be when I grow up.

Correct answer: (d) what

Your answer was: incorrect
This is why I want to be when I grow up.
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could you please explaine me why answer is what and not why.
thank you.
Isram

[Mister Micawber]

.
'Why I want to be' simply makes no sense, Isram, outside of a philosophical discussion. 'What I want to be' means 'what career I want to pursue'.
.
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Native English teacher at Mister Micawber's