Fortunately, there is a simple way to evaluate this hypothesis, at least on a preliminary basis, prior to waiting for the results of longitudinal studies. Enough youth are experimenting with electronic cigarettes, and the increase in youth e-cigarette use has been dramatic and sustained enough, that if the gateway hypothesis were correct, we would be starting to see an increase in smoking prevalence among youth due to the many new smoking initiates that were converted from e-cigarette experimentation to smoking.

F. None of the above.

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