Help with Simplifying Radical Expression

Hello, I need help with simplifying the radical expression: sqrt10/81? It's a radical sign and the fraction 10/81 under it. I have two choices: 10/9, which makes the most sense but does not equal the original expression and 9sqrt10 which makes more sense because it equals the original expression, but exactly how did they arrive there and why is it right?

Thanks.

neither choice is correct
the correct answer is (√10) / 9