Below is an attempted proof that is unsuccessful, due to more than one error. Consider the proof and read the question carefully, then fill in the blank with the answer.

Consider the wff from the last question. We needed that wff in order to use MT with Line $2.$

What rule would justify our writing that wff we need on Line $5?$ In other words, to make MT work, we need a different wff$.$ What rule would produce that wff from one that we already have?

$($Don$$t worry about the line #$$s$,$ just write the rule$$s name$/$abbreviation$.)$

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

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

Aim:

$($Q $->$ S$)$

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

$3(3)$ Q A$/$CP $|-$ S

$1,3(4)$ R $1,3$ MP