A savings plan pays $7\mathrm{.}5\%$ compounded quarterly. Paul deposits $ $500$ in this account at the end of every month, for $10$ years. Find c $($the number of interest conversion periods per period interval$)$a$\mathrm{.}1/6$b$\mathrm{.}1/3$c$\mathrm{.}3$d$\mathrm{.}4$e$\mathrm{.}6$