PrepTest 145, Section 4, Question 23

What we’re looking for

Working the answers

• A

  Here the three categories cover every vaccine, and vaccine X is already identified as a vaccine. Two categories are eliminated, so the argument concludes outright that X belongs to the third. The original does not conclude outright that the computer can learn from mistakes; it retains the unestablished condition that the computer is intelligent. A lacks that same conditional conclusion.

• B
  Correct

  Being commonly used plays the role of being intelligent. Dead-virus, attenuated-virus, and pure DNA are the three alternatives, like creativity, self-awareness, and learning from mistakes. X is not in the first two categories, just as the AR3000 lacks the first two qualities.

  The conclusion preserves the right condition: if X is commonly used, it must be pure DNA. Neither argument establishes that its subject belongs to the special group. Each tells us which remaining feature the subject must have if it does belong.

• C

  This rule covers every vaccine, and X is a vaccine. The uncertainty in the conclusion concerns whether the first two categories are absent: if X isn't dead-virus or attenuated-virus, then it is pure DNA. In the original, the first two qualities are already known to be absent; what remains conditional is the computer's intelligence. C puts the condition in a different place.

• D

  The T-cell information does rule out pure DNA: X has a property no pure DNA vaccine has. We shouldn't reject that information just because it sounds technical. But only one category has been ruled out as a fact; the absence of dead virus appears in the conclusion's “if” clause.

  More importantly, the three-category rule applies to commonly used vaccines, and D never establishes that X is commonly used or makes its conclusion conditional on that. Even after excluding two categories, the rule cannot force the third unless the common-use condition is met. The original keeps its corresponding intelligence condition.

• E

  The fact rules out attenuated virus, and the conclusion asks what follows if dead virus is also ruled out. That divides the two exclusions between a fact and a condition, unlike the original's two stated exclusions. It also omits the crucial condition that X is commonly used. X might fall outside that group, in which case the three-category rule doesn't determine its type.