Be-to infinitive (why used here)

[Duc]

Hello,
Below is a sentence rewriting exercise:
1/He met Jane,whom he later married,when he was at Cambridge.
==> He met Jane,who ............
the correct answer is ''He met Jane,who was later to become his wife ,when...''
I know there is a construction with ''Be-to V'' in English but in this context I wonder why they don't use ''who later became his wife'' instead of ''was later to become his wife''?
Help me please.
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Don't see your shade think you are great

[Haihao]

I think both are fine but ''was later to become his wife'' has a better rhythm. I am sorry I can't explain more.

[Duc]

I want some more helps.Help me please !
Thanks in advance!
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Don't see your shade think you are great

[Nessie]

I agree with Haihao: both versions are correct. But because your book can't give all the possible correct answers, it gives only one, and that doesn't means other ways of tranforming the sentence are incorrect, right?
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... something we never have again, I know... I guess I really really know..

Sorry seems to be the hardest word...

[Duc]

yes,in Vietnam it is extremely odd that there is only one correct answer to a question, other possible answers are either reluctantly accepted or not allowed to use.
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Don't see your shade think you are great