Which of the following is correct expression for the $C$ code:

$d=e-f$

where d$,$ e and $f$ are in $$s1,$$$s2$ and $$s3,$ respectively

sub $$s3,$$$s2,$$$s1$

sub $$s1,$$$s3,$$$s2$

sub $$s2,$$$s3,$$$s1$

sub $$s1,$$$s2,$$$s3$