Considering the way that this analysis was carried out, which of the following should be said about the population to which the findings can generalize?

$($a$)$ The population is all individuals who buy the type of products sold on the retail app.

$($b$)$ The population is all individuals with a smartphone.

$($c$)$ We should be careful to extend the findings to all potential users of this retail app, because the sample only included individuals who voluntarily downloaded the beta version of the app.