$\begin{aligned} 23 \times 64 = 32 \times 46 \\ 36 \times 42 = 63 \times 24 \\ 26 \times 31 = 62 \times 13 \end{aligned}$

We call a pair of 2-digit positive integers *reversible* if their product remains the same when we reverse the digits in both integers.

The above shows that the pairs $(23,64), (36,42), (26,31)$ are reversible.

Which of the following is **not** reversible?