Statistics about sexual activity have suggested that:
a. more people engage in oral intercourse than vaginal intercourse
b. the majority of people engage in heterosexual, vaginal intercourse in the context of a relationship with one partner
c. $10 \%$ to $15 \%$ of the population has exclusively homosexual sex
d. in the 1990s, people were more likely to be having sex with multiple partners than they were to be in a monogamous sexual relationship