I know that people emphasize idioms a lot, but guess what, don't look for them too sharply--the only sort of idiom that the test likes to test is prepositional ones, and those, appear only once (at most, twice!) on the test.
Don't flip out over them!
(I thought "Attaining the speed of 60 miles per hour" should be changed to "Reaching the speed of...," apparently not. There are a whole bunch of other examples I could cite, but just don't bother correcting things that are awkward. They're usually right.)