The $9\text{'}$s complement of a decimal digit d is defined to be $9-$d$.$ Give the representation of $d=7$ and the $9\text{'}$s complement of $d$ if the decimal digits are represented in

a$)BCD$

b$)$ Excess$-3$

c$)6311$ code

d$)$ ASCIl code

What property does the Excess$-3$ code have that BCD doesn't?