Automobile-emission standards are enforced through annual inspection. At those inspections cars are tested while idling; that is, standing still with their engines running. Testing devices measure the levels of various pollutants as exhaust gases leave the tail pipe. Which one of the following, if true, most strongly indicates that current enforcement of automobileemission standards might be ineffective in controlling overall pollutant levels? ['The testing devices used must be recalibrated frequently to measure pollutant levels with acceptable accuracy.', 'As an emission-control technology approaches its limits, any additional gains in effectiveness become progressively more expensive.', 'When emission standards are set, no allowances are made for older cars.', 'The adjustments needed to make a car idle cleanly make it likely that the car will emit high levels of pollutants when moving at highway speeds.']