Researchers in Australia experimented to determine if color of a coffee mug affects how people rate the flavor intensity of the coffee. Volunteers were randomly assigned to taste coffee in mugs: some white and some clear. If same type of coffee was used, researchers concluded that rating was significantly higher for those who drank coffee in clear mug. What can be concluded.
A) Color caused the difference and can be generalized to all drinkers
B)Same as A but cannot be generalized to all drinkers

Volunteers are not a random sample, so the results cannot be generalized to all coffee drinkers. There may be something different about people who would volunteer for a coffee drinking study. For example, people who would volunteer for such a study might be more likely to drink a lot of coffee and thus consider themselves able to discern subtle differences in taste.