题目选项分析

Studies have shown that an automobile that runs on a blend of 85% ethanol/15% gasoline gets better mileage than an otherwise similar car equipped with a gasoline engine. Many American legislators have concluded that an increase in tax incentives for ethanol production would lessen our dependence on foreign oil.

Which of the following, if true, casts the most doubt upon the validity of the legislators' conclusion?

【选项】It costs thousands of dollars to retrofit an automobile to run on the ethanol fuel mix.