Let $S$ be the set of all bit strings $($strings of $0\text{'}$s and $1\text{'}$s$)$ of length at least $2.$ Which of the following functions $f$:$SS$ is NOT one$-$to$-$one?

A$),f(s)=$ the string $s$ with a $1$ bit appended at the end. $($For example, $f(1101)=11011.)$

B$),f(s)=$ the reversal of For example, $f(110)=011.$

$f(s)=$ the string obtained from $s$ by interchanging $0\text{'}$s and $1\text{'}$s$.($For example, $f(11000)=00111.)$

D$),f(s)=$ the string obtained by moving all $0\text{'}$s $($if any$)$ in $s$ to the end of the string. $($For example, $f(101101)=111100.)$