Is it really true that GMAT does not look into the idiomatic preference.
As per GMAT's directive, it is very unlikely that a sentence would only
test you on idioms; in other words, idiomatic usage
is very unlikely to be the only difference between a correct choice and an incorrect one.
Having said that, there are a couple of questions in OG-13 that actually do test you purely on idiomatic usage
(let me know if you want to know which one; I can pull it out). However, it does look unlikely to be the case in the actual exam.
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