If we have six stack operationspushing and popping each of $A, B$ and $C$-such that push $(A)$ must occur before push $(B)$ which must occur before push $(C)$, then $A, C, B$ is a possible order for the pop operations, since this could be our sequence : push $(A)$, pop $(A)$, push $(B)$, push $(C)$, pop $(C)$, pop $(B)$. Which one of the following orders could not be the order the pop operations are run, if we are to satisfy the requirements described above?
- $ABC$
- $CBA$
- $BAC$
- $CAB$