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The question is stating that the body’s immune system requires numerous infections to be properly stimulated. It is a causal argument that tries to explain an observation.

Premise #1: Children tend to get fewer colds as they progress through pre-school.
Premise #2: (unstated assumption)
Conclusion: It takes several colds to activate a child’s immune system.
Analysis: This is an After This, Therefore, Because of This fallacy. It observes that as children go through pre-school the number of colds goes down. From this, the creative author develops the theory that a child’s immune system requires them to get several colds before it is fully activated.

The best way to weaken a causal argument is to suggest an alternative causal factor.

Reviewing Answer Choices

(A) Parents stock up on cold medicine that alleviates the symptoms of a cold after a child gets sick. This choice presents another possible reason to undermine the argument, but the medicine deals with symptoms, not the cold per se. So it is not reducing an instance of a cold but simply decreasing its symptoms (no more runny noses!).

(B) There are many strains of the cold virus and children develop resistance to individual strains. This choice suggests an alternative explanation for the apparent improvement in a child’s ability to fight colds: the child simply becomes immune to individual viruses. So, the theory that a child’s immune system needs high white blood cell concentrations isn’t the case; it is an issue of exposure to certain strains. By suggesting a different causal process to explain the reduction in colds, it weakens the argument.

(C) White blood cells fight infection, and their production levels are stimulated by high infection levels.This choice supports this statement, but the question asks for what weakens it.

Trick opposites are sometimes used as junk answer choices on Strengthen/Weaken questions. If the stem asks for what weakens the passage, you’ll find a perfect answer choice for what strengthens it, and vice versa. Choice (C) in the above question about colds is an example of a trick opposite.