Great explanation, jaymw, although I may suggest that you do it in a slightly different order...
>99% of you will never see the idiom "superior to" vs. "superior over" on the GMAT, so it's not really worth spending any time on. You're much more likely to swim in Lake Superior than to have to know any idioms pertaining to the word superior!
And you don't need it here. First, look at the fact that three choices use "and" and two do not. Do we need to connect "of the motorcar was demonstrated" to "its superiority"? Yes - otherwise we're just listing nouns out of context, so D is incorrect. E uses a connector so for the time being it's okay, but next let's look at verb tense, since we can see "being", "had been", etc. all used.
Because we're using parallel verbs (Once X and Y - they happen at the same time), we need to have "was", and only C uses the proper tense (which in this case is implied). A, B, and E all use incorrect verb tenses, and are therefore wrong. Only C, the correct answer, remains.
Now, it's true that C gets the idiom correct, but on the GMAT there are very few questions on which you'll have to know the idiom, so I'd look at those as "bonuses" - if you know them, great, but don't stress them if you don't. The test will reward you for systematic thinking, so focus on practicing those major errors that you know you can do well.
Brian Galvin
GMAT Instructor
Chief Academic Officer
Veritas Prep
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