Researchers Wish to Conduct an Experiment Investigating the Effects of Smoking

Researchers wish to conduct an experiment investigating the effects of smoking on math performance.They place an ad in the national paper and invite smokers of all ages to participate.Such a process allows

A) only smokers with lung cancer to participate in the study.
B) the researchers to find the relationship between smoking and math ability.
C) only the smartest participants to apply for the study.
D) the researchers to generalize the results to the population.