Doug Notices in His Sample Studies That Marijuana Users Are

Doug notices in his sample studies that marijuana users are much more likely to report using heroin than people who do not use marijuana. He concludes that marijuana must be a gateway drug-a drug that leads to the use of harder substances. What's wrong with Doug's conclusion?

A) His sample data do not demonstrate causality; an experimental approach would be better for investigating such a claim.
B) Marijuana users cannot be trusted to report their behavior honestly and accurately.
C) Doug could never really know without trying marijuana himself and seeing whether it leads to heroin use.
D) Marijuana users would likely be afraid of needles because they are used to smoking their drug of choice.