Hi Skywalker,
The first sentence explains the basic idea using last year's data: (X) many new jobs in new companies + (Y) many new jobs in exiting companies = (Z) record total new jobs.
The next sentence says that this year, the number of new jobs from existing companies (Y) will not be as great as it was last year. The fact that this year's exiting companies are last year's new companies does not affect the formula. It's only there to confuse you.
The last sentence is the conclusion: unless there's a record number of new companies, the total new jobs this year (Z) will be less than last year's.
This seems to make sense, because we know this year (Y) is lower, so it's all about (X). The assumption lies in the author's treatment of (X): the author incorrectly equates the number of new companies with the number of new jobs in new companies (X). This is the assumption - it is perfectly plausible that a few new companies will offer lots of new jobs, and the author overlooks this possibility.
Now to find an answer choice that reflects this assumption: D and E talk about lost jobs, that's beside the point. A talks about the ratio between (X) and (Y) but that's not the assumption. B is attractive because it refers to the confusing effect mentioned earlier - the companies which were new last year and are now established - but it doesn't explain the conclusion. C is correct because it shows what the author believed - that few companies cannot make many more jobs, i.e., that the number of new jobs per company isn't significantly greater this year.
May the force be with you.
Amit Moshe
Verbal Section Instructor
Economist GMAT
