Consider achieving the best performance for the following instructions:

$1$ Add $R4,R2,R3$

$2.$ SW RS$,$ #$100($R$4)$

$3.$ LW R$6,$ #$200($R$4)$

$4.$ Sub R$7,$ R$5,$ R$6$

the data hazard between $2$ and $4$ are solved by:

Select one:

a$.$ forwarding from Ex in $2$ to Ex in $4$

b$.$ forwarding from Ex in $2$ to mem in $4$

c$.$ forwarding from Mem in $2$ to mem in $4$

d$.$ stall

e$.$ no data hazards