In research studies, which situation demonstrates self-selection bias?

Select one:

a. In an online survey on gun control, gun owners were twice as likely to respond as non-owners.

b. For a statewide poll, a newspaper only contacted residents of urban areas.

c. In an aging study, men were more likely than women to die before study completion.

d. In an experiment, most of the female participants were assigned to the control group by chance.