Numerous studies have demonstrated a pronounced negative correlation between high-fiber diets and the incidence of colon cancer.
Conclusion
Insufficient fiber consumption causes colon cancer, and sufficient consumption prevents it.
Evidence
Several studies show a negative correlation: high-fiber diet || lower incidence of colon cancer
Scandinavia: higher colon cancer || less cereal eating (cereal is high fiber)
Evaluate
Well the conclusion is way too strongly worded. Correlations between X and Y strengthen the plausibility that X causes Y, but they don't prove it (maybe Y causes X / maybe some third factor explains the correlation).
The first correlation could be vulnerable to reverse causality (maybe people who have colon cancer are put on low fiber diets). The Scandinavian finding could be easily weakened if we were to learn that even though they don't eat as much cereal, they eat way more fruits and veggies than usual (so they actually do have plenty of fiber in their diets).
Goal
It's hard to predict which angle the correct answer will take, but it will be some form of Causal-weakener that either - provides a different way to explain the correlation or - undermines the plausibility that eating fiber is good for preventing colon cancer.