Why not use 'which' instead of 'what' here

[Stone]

English Grammar Tests, Elementary Level

ESL/EFL Test #93 "Why Questions", question 6

......... year did Prince Charles of England marry?

(a) What
(b) Which
(c) Where
(d) Who

English Grammar Tests, Elementary Level

ESL/EFL Test #93 "Why Questions", answer 6

What year did Prince Charles of England marry?

Correct answer: (a) What

Your answer was: incorrect
Which year did Prince Charles of England marry?
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Dear Sir

Would you please explain why not use 'which' instead of 'what' here?

thanks a lot in advance

Stone

[Mister Micawber]

.
We expect to use what in this question because, for the average reader, there is a wide range of years as possible answers. I myself have no idea as to what decade saw his marriage!

Which is normally reserved for the cases of two only or a very limited number of choices:

What time is it?
Which time is better for you, 2:30 or 3:30?
.
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Native English teacher at Mister Micawber's