It Is Difficult to Establish the Causal Link Between Cigarette

Multiple Choice

It is difficult to establish the causal link between cigarette smoking and lung cancer because

A) random allocation of subjects to smoking is unethical.
B) observational studies generally cannot rule out confounding.
C) those who choose to smoke may be genetically at greater risk for lung cancer than those who don't choose to smoke.
D) experiments done on animals may not be valid for humans.
E) All of the above