Thinking about bonds and Treasury bills, which one of the following statements is correct?

a$.$ A Treasury bill that repays $100,000$ at the end of $91$ days is sold at a discount of $1525.$ The annual yield on this bill is $5\mathrm{.}21\%($to two decimal places$).$

b$.$ If the discount at which a Treasury bill is sold increases, the bill's yield falls.

c$.$ Ignoring any gain or loss when the bond is sold or redeemed, the yield on a bond with a nominal value of $100,$ a coupon of $2\mathrm{.}5\%$ and a market price of $97\mathrm{.}50$ is $2\mathrm{.}56\%($to two decimal places$).$

d$.$ A central bank can increase the yield on bonds by using a policy of quantitative easing.