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

Which of the following changes will suffice to make this a prefix$-$free code?

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

Change the code for $a$ to $110.$

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

Change the code for $h$ to $1100.$

Change the code for $t$ to $1110.$