The link between smoking and cancer, for example, was seen in many observational studies, but forcing subjects to smoke for years to prove it would be untenable.
3
事实上出现预期寿命下降这样的趋势,罪魁祸首是与吸烟和肥胖有关的肺癌和糖尿病所致。
The main culprits are diseases linked to smoking or obesity, such as lung cancer and diabetes.