Below is the beginning of a proof. Consider the proof and read the question carefully, then fill in the blank with the answer.

This proof is set up to start, with the given premises and the aim box filled out already. What rule should we use on the next line of our proof in order to proceed? $($HINT: It is not Conditional Proof...$)$

$($Q $->$ R$),($S $->$ T$)|-(($S & Q$)->($R & T$))$

$1(1)($Q $->$ R$)$ A

Aim:

$(($S & Q$)->($R & T$))$

$2(2)($S $->$ T$)$ A