Consider the following actions that a central bank can take with regard to the size of its

balance sheet:

$($I$)$ increasing purchases of mortgage$-$backed securities and longer$-$term Treasury securities

$($II$)$ selling off mortgage$-$backed securities and longer$-$term Treasury securities

$($III$)$ repurchasing new mortgage$-$backed securities and longer$-$term Treasury securities when

existing securities reach their maturity

Which of the above actions would facilitate unwinding Quantitative Easing?

$($A$)$ I only

$($B$)$ II only

$($C$)$ III only

$($D$)$ I and III only