def choice(arr, n)
　arr.select { |x| x % (n + 1) == 0 }
end

a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
choice(a, 1) => [0, 2, 4, 6, 8]
choice(a, 2) => [0, 3, 6, 9]
choice(a, 3) => [0, 4, 8]