In a long-distance road test, two cars of different makes were driven on the same 600-mile route. One of the cars, a gasoline (petrol) powered car, used more fuel on the route than did the other, a diesel powered car. Clearly, the diesel engine is more fuel-efficient than the gasoline engine.
The conclusion of the argument is the following:
the diesel engine is more fuel-efficient than the gasoline engine
The reasoning of the argument is that, since the two cars were driven on the same route and the gasoline powered car used more fuel than the diesel powered car did, the diesel engine is more fuel-efficient.
Which of the following would, if true, most strengthen the argument?
This is a Strengthen question, and the correct anwer will strengthen the support provided or provide additional support for the conclusion.
A. The diesel powered car is a model that is especially popular because of its fuel economy.
The fact that the diesel powered car is popular because of its fuel economy doesn't mean it's more fuel-efficient than the gasoline powered car.
After all, the gasoline powered car could also be fuel-efficient and could also be popular because of its fuel economy.
Eliminate.
B. Most cars of the same make as the diesel powered car have gasoline engines, not diesel engines.
This information about what engines cars have doesn't indicate anything about how fuel-efficient the engines are.
Eliminate.
C. The diesel powered car encountered greater wind resistance on the route than did the gasoline powered car.
This choice strengthens the argument by serving to rule out a variable that could have skewed the results of the road test in such a way as to support an incorrect conclusion.
After all, it could have been that the reason the diesel powered car used less fuel was not that it is more fuel-efficient but that it encountered less wind resitance than the gasoline powered car. This choice serves to rule out that possibility.
In other words, if, as this choice says, the diesel car encountered greater wind resistance than the the gasline powered car, then we know for sure that the diesel car didn't use less fuel merely because it was helped by the wind. So, given what this choice says, we have more reason to believe that the diesel car is indeed more fuel-efficient than the gasoline powered car.
Keep.
D. Fuel economy for all cars varies greatly by speed because of wind resistance.
This general fact about fuel economy "for all cars" does not indicate anything about which of the two cars tested is more fuel-efficient.
Eliminate.
E. The gasoline powered car encountered greater wind resistance on the route than did the diesel powered car.
This choice is the opposite of (C). (I don't think I've ever before seen in a Strengthen question two choices that are opposites or conflict.)
So, whereas choice (C) strengthens the argument, this choice weakens it.
After all, if this choice is true, then there's a possible alternative explanation for the fact that the diesel powered car used less fuel. It could be that the gasoline powered car used more fuel because it was dealing with more wind resistance.
That information would cast doubt on, rather than support, the conclusion that the diesel engine is more fuel-efficient.
Eliminate.
Correct answer: C