Why the prep "at" isn't suitable?

[Hoamuahe]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #664 "Prepositional Phrases - At, By, For and To (2)", question 4

I just bought this tool set ......... a fraction of its regular price. Waiting for a sale to take place, rather than buying regular-priced tools, is a good way to save a lot of money.

(a) at
(b) by
(c) for
(d) to

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #664 "Prepositional Phrases - At, By, For and To (2)", answer 4

I just bought this tool set for a fraction of its regular price. Waiting for a sale to take place, rather than buying regular-priced tools, is a good way to save a lot of money.

Correct answer: (c) for

Your answer was: incorrect
I just bought this tool set at a fraction of its regular price. Waiting for a sale to take place, rather than buying regular-priced tools, is a good way to save a lot of money.
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why the prep "at" isn't suitable ?

[Linda]

Hi,
You are right about this question because both prepositions are correct. We will rework this question so there is only one possible answer. Good catch!
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