Driving excessively fast has been demonstrated to decrease the number of miles one can drive per gallon of fuel. Gary has recently been experiencing a decrease in mileage per gallon of fuel while driving his car. This clearly proves that Gary has been driving excessively fast lately.
Which of the following statements, if true, would most seriously weaken the conclusion about Gary’s driving?
(A) Recently Gary’s speedometer has been indicating the speed of his car as lower than the car’s actual speed.
(B) Recently Gary has been driving more miles per day on average than before he began experiencing a decrease in fuel mileage.
(C) Other tests have shown that a car’s speed affects fuel mileage more than any other single factor.
(D) Before Gary began driving excessively fast his speedometer over-represented his car’s actual speed.
(E) Recently the tires on Gary’s car have been losing air pressure, and low tire air pressure is known to lower fuel mileage.
The argument relies on the unstated assumption that no factor other than Gary’s driving speed might be responsible for the recent decrease in his fuel mileage; in other words, no other circumstances that might affect fuel mileage have changed recently. One effective way to weaken the argument would be to refute this assumption. Choice (E) accomplishes this by providing a convincing alternative explanation for the decrease.