Critical Reasoning

A significant number of complex repair jobs carried out by Ace Repairs have to be reworked under the company's warranty. The reworked jobs are invariably satisfactory. When initial repairs are inadequate, therefore, it is not because the mechanics lack competence; rather,there is clearly a level of focused concentration that complex repairs require that is elicited more reliably by rework jobs than by first-time jobs. The argument above assumes which of the following?

A. There is no systematic difference in membership between the group of mechanics who do
first-time jobs and the group of those who do rework jobs.
B. There is no company that successfully competes with Ace Repairs for complex repair jobs.
C. Ace Repairs' warranty is good on first-time jobs but does not cover rework jobs.
D. Ace Repairs does not in any way penalize mechanics who have worked on complex repair jobs
that later had to be reworked.
E. There is no category of repair jobs in which Ace Repairs invariably carries out first-time jobs
satisfactorily.

The official answer is A but isnt A already stated in the argument i.e the bold part. Please help.