A bank offers your firm a revolving credit arrangement for up to $$110$ million at an interest rate of $2\mathrm{.}1$ percent per quarter. The bank also requires you to maintain a compensating balance of $7$ percent against the unused portion of the credit line, to be deposited in a non$-$interest$-$bearing account. Assume you have a short$-$term investment account at the bank that pays $1\mathrm{.}4$ percent per quarter, and assume that the bank uses compound interest on its revolving credit loans.

a$.$ What is your effective annual interest rate $($an opportunity cost$)$ on the revolving credit arrangement if your firm does not use it during the year?