If \(x\) and \(y\) are positive integers and \(n=5^x+7^{y+15},\) what is the unit's digit of \(n?\)

[Gmat_mission]

If \(x\) and \(y\) are positive integers and \(n=5^x+7^{y+15},\) what is the unit's digit of \(n?\)

(1) \(y=2x–15\)

(2) \(y^2–6y+5=0\)

[spoiler]OA=B[/spoiler]

Source: Manhattan GMAT