Which of the following is an example of a dose-response relationship?

a. As the length of smoking history increases, so does the risk of lung cancer
b. Smokers have ten-fold increased risk of lung cancer compared to non-smokers
c. In a conditioned reflex experiment, rats took a dose of nicotine when a bell rang
d. Onset of smoking usually precedes clinical onset of lung cancer by 20-30 years
e. After approximately fifteen years, smokers who quit have a risk of lung cancer that is almost as low as that in never-smokers