Consider the following variable$-$length code for the following eight letter alphabet:

Which of the following changes will make this a prefix$-$free code? $($Pick $a$ if no changes are

necessary.$)$

None $($It is already a prefix$-$free code$).$

Change the code for $a$ to $1101.$

Change the code for $j$ to $1101.$

Change the code for $m$ to $100.$

Change the code for $s$ to $01010.$