clearly "A" - just negate the satement and you would see how conclusion falls apart.
If there is altogether a different worker group who contribute to rework jobs than the cause is lack of mechanics competence and not amount of concentration.
Whatz wrong with D?
"D" is wrong bcoz penalizing workers for rework is just out of scope. moreover, negation of the same wont destroy the conclusion
" 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. "
hope this will help
Perseverance, Hard Work and self confidence