$39.$ suppose you have the instruction add R$1,$ R$2,$ R$3$ using one bus organization. Suppose that you have $8$ general purpose registers $($RO$-$R$7)$ and $4$ special purpose registers $($IR$,$ PC$,$ MAR, MDR$),$ the ALU functions are $4($add$,$ subtract, multiply, and divide$).$

When using the horizontal implementation, then we need

a$.12$ bits for source$1.$

b$.13$ bits for source$1.$

c$.14$ bits for source$1.$