题目选项分析

Companies that offer "employer sponsored insurance" (ESI) pay a portion of employees` health care costs. In the manufacturing sector last year, companies that offered ESI had worker absentee rates 22% lower, on average, than those at companies that did not offer ESI.

If, on the basis of the evidence above, it is argued that ESI decreases worker absenteeism, which of the following, if true, would most seriously weaken that argument?

【选项】In manufacturing firms where ESI is offered, the average productivity is 2% higher than it is in those firms where workers are not covered by an ESI plan.