Please include an explanation

In order to study the effect of smoking on incidence of stroke, a doctor collected data about the number of cigarettes men smoke per day. The men he randomly chose came from two groups: those who had had strokes and those who had not. Which hypothesis test would be the best one for him to use? O One mean, sigma known. O One mean, sigma unknown. O Mean of differences, using dependent samples (paired data). Difference of two means, using independent samples. O ANOVA