Passage Summary: Younger kids with lots of siblings get fewer allergies, so researchers think catching germs early in life actually helps the immune system.

Conclusion: Early exposure to germs is the reason children in large families are less likely to develop allergies.

Reasoning: Data shows that children from large families, particularly younger siblings who interact with older ones, have fewer allergies than children from small families.

Analysis: To strengthen this causal hypothesis, we need to reinforce the connection between 'large families' and 'increased germ exposure.' The researchers are assuming that the reason these kids have fewer allergies is specifically the germs, rather than some other factor like genetics or diet common in large families. Look for an answer that provides more evidence for the 'hygiene hypothesis,' perhaps by showing that other children exposed to high levels of germs also have lower allergy rates. Strengthening the link between the proposed cause and the observed effect is key here.