How to use the conditionals in English?

[Guest]

Test No. incompl/elem-11 "Conditionals", question 1

If I win the lottery, I ......... buy you a very big car as a present.

(a) should
(b) will
(c) am

Test No. incompl/elem-11 "Conditionals", answer 1

If I win the lottery, I will buy you a very big car as a present.

Correct answer: (b) will

Your answer was: correct
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why will 'will' be used

[Alan]

This is a so-called possible condition sentence - with Present and Future constructions. Have a look on this site under the heading ESL LESSONS and half way down the list you will see the word Conditionals, which is a story I've written to show the different forms.
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English as a Second Language
You can read my ESL story Guy Fawkes Night for You

[Luby]

I wrote 'should' but its wrong,it says that 'will' is the correct one?
Why will why not should?