$89\div9=9R8$ 
When number $\overline{OK}$ is divided by $K$, the result will give the same quotient as the divisor and leave the remainder of $O$. The same letter represents the same digit, and different letters represent different digits. Then, what is the product of the two digits, $O$ and $K$? 
[1] $$18$$ 
[2] $$36$$ 
[3] $$63$$ 
[4] $$72$$ 
[5] $$81$$ 