---

        // 21а
        Console.Write("Введите слово: ");
        string s21a = Console.ReadLine();
        int idx21a = -1;
        for (int i = 0; i < s21a.Length; i++)
        {
            if (s21a[i] == 'о')
            {
                idx21a = i;
                break;
            }
        }
        if (idx21a == -1)
        {
            Console.WriteLine("Буква о не найдена");
        }
        else
        {
            char[] chars21a = s21a.ToCharArray();
            for (int i = idx21a; i < chars21a.Length - 1; i++)
                chars21a[i] = chars21a[i + 1];
            chars21a[chars21a.Length - 1] = '_';
            Console.WriteLine(new string(chars21a));
        }