A passenger vehicle is defined as any fully-enclosed four-wheeled vehicle with room for at least one other person besides the driver. A semi truck, though it does have enclosed room to transport one non-driving passenger, always has more than four wheels. A jeep, though it may have four wheels and enough room for a driver plus at least one passenger, is rarely fully enclosed. A postal van, though it may be enclosed and have four wheels, usually does not have room for any passengers other than the driver.
If all of the above statements are true, which one of the following MUST also be true?
A. If an automobile is neither a semi truck, nor a jeep, nor a postal van, then it is not a fully enclosed four-wheeled vehicle.
(A) is incorrect because it could obviously still be a passenger vehicle, which according to the stimulus is a fully-enclosed four-wheeled vehicle. Of course it could be something else, meaning it COULD be an open vehicle and/or one with more or fewer than four wheels, but it doesn't HAVE to be.
B. If an automobile is not a semi truck, or a jeep, or a postal van, then it must be a passenger vehicle.
(B) is incorrect because the stimulus leaves open the possibility that there are more types of automobiles besides the four that are described. We can reach this conclusion logically, regardless of our outside knowledge about automobiles, because there is no language in the stimulus that tells us an automobile MUST or CAN ONLY be one of the four types described.
C. If a jeep is fully enclosed with four wheels and room for non-driving passengers, then it is both a jeep and a passenger vehicle.
(C) is correct. The key to answering a question like this is being able to determine, and more importantly to distinguish, what MUST be, what COULD be, and what CANNOT be. All of the descriptions in the stimulus contain qualifiers that indicate whether the particular characteristic is ALWAYS true, USUALLY true, RARELY true, or NEVER true of that type of vehicle. If something is true "usually," "rarely," "sometimes," "often," "seldom," etc., that means it CAN be true but it doesn't HAVE to be true, and it can also be false but doesn't have to be false. The stimulus tells us that a jeep is "rarely fully enclosed," not that it is never fully enclosed, which means it CAN be fully enclosed. Since the question contemplates a fully-enclosed jeep, and since any fully-enclosed vehicle with four wheels and room for two or more people is a passenger vehicle, then such a vehicle would be both a jeep and a passenger vehicle. Even if the stimulus told us that a jeep was never fully enclosed, this would still be the correct answer because the answer itself contemplates a fully-enclosed jeep, and what such a vehicle would be if it did exist.
D. Postal vans are more like passenger vehicles than semi trucks are.
(D) is incorrect not because it is a matter of opinion, but because there are no objective, logical criteria for comparison, i.e., whether having the same number of wheels makes a vehicle "more like" another than if it had the same amount of passenger room. Postal vans are similar to passenger vehicles in that they tend to be enclosed and have four wheels, but they have no room for passengers. Semi trucks are like passenger vehicles in that they are enclosed and have room for passengers but they have more than four wheels. Each compared vehicle therefore shares two of the three stated characteristics with passenger vehicles, but we don't know which characteristic is more important, so it is impossible to say that either is definitively "more like" a passenger vehicle than the other.
E. Some postal vans may be considered to be jeeps.
(E) is incorrect because the descriptions of both jeeps and postal vans are so equivocal that even if the categories could overlap, it is also possible that they don't, i.e., that no postal vans are jeeps. The facts that jeeps are RARELY enclosed (which postal vans always are) and that postal vans USUALLY have no passenger room (which jeeps always do), suggest that it's unlikely that a postal van is also a jeep, but not impossible; still, we cannot conclude with any certainty that there must be postal vans out there that are also jeeps. Maybe there are, maybe there aren't; remember the question is what MUST be true, not what COULD be true. (Note also the use of the phrase "may be," as opposed to "can be" or "could be." As you know from elementary school, "may" implies permission, not possibility; i.e., that such vehicles DO exist, not that they CAN exist.)