Which of the Following Methods Is the Correct Way to Calculate

Which of the following methods is the correct way to calculate a cigarette smoker's pack-year?

A) Number of packs of cigarettes smoked per week multiplied by the number of years smoked
B) Number of packs of cigarettes smoked per day divided by the age of the patient in years
C) Number of packs of cigarettes smoked per day multiplied by the number of years smoked
D) Number of packs of cigarettes smoked per year divided by the number of years smoked
E) Number of packs of cigarettes smoked per month multiplied by the number of years smoked