Correct Response: D. Asthma is a lung disease that causes airways to constrict, leading to difficulty in breathing. Asthma attacks can be triggered by chemicals, dust, and/or air pollution. An increase in the number of patients being treated for asthma in medical facilities near the chemical factory would suggest a causal relationship, especially if the increase in asthma patients coincides with activity from the chemical factory. Although visible air pollution near the factory (A) suggests that the factory may be affecting the local environment, this observation does not provide evidence about how people's health has been affected. Historical links between chemical manufacturing and human health issues (B) signal that health issues are a potential risk but do not provide direct evidence that health issues are occurring around this specific chemical factory. Although opinions (C) may be based on observations, opinions themselves are subjective and not the best source of information on which to determine a causal relationship.
