I prefer e-mail to fax machines

[Taisfatel]

Business Idiom in English, Intermediate level

ESL/EFL Test #111 "Faxes", question 7

I prefer e-mail ......... fax machines because it is so much less complicated.

(a) instead
(b) than
(c) to
(d) of

Business Idiom in English, Intermediate level

ESL/EFL Test #111 "Faxes", answer 7

I prefer e-mail to fax machines because it is so much less complicated.

Correct answer: (c) to
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why is it the correct answer "to" instead of "instead"?

[Beeesneees]

At the very least, to use 'instead' you also need 'of':
I prefer e-mail to fax machines...
I prefer e-mail instead of fax machines...

However, 'instead of' would not represent the same vocabulary choice in this sentence, because 'instead' means 'as an alternative to' rather than 'as a comparison with', which is what 'to' does here (introduce the second part of a comparison).
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Cheers m' dears!