Which of the following, regarding social trust, is true?

a. Social trust is positively correlated with GDP per capita and negatively correlated with income inequality across countries.

b. Stolle and Harell found ethnic diversity of one's social network positively associated with trust among older Canadian adults, but negatively associated with trust among Canadian youth.

c. Interpersonal trust has been roughly constant in the U.S. since 1960.

d. Religious participation is associated with lower levels of social trust, on average.