---

        // 24
        Console.Write("Введите предложение: ");
        string s24 = Console.ReadLine();
        char[] chars24 = s24.ToCharArray();
        int write24 = 0;
        for (int i = 0; i < chars24.Length; i++)
        {
            if (chars24[i] != 'с')
            {
                chars24[write24] = chars24[i];
                write24++;
            }
        }
        for (int i = write24; i < chars24.Length; i++)
            chars24[i] = '_';
        Console.WriteLine(new string(chars24));

        // 25
        Console.Write("Введите слово: ");
        string s25 = Console.ReadLine();
        char[] chars25 = s25.ToCharArray();
        int write25 = 0;
        for (int i = 0; i < chars25.Length; i++)
        {
            bool found25 = false;
            for (int j = 0; j < write25; j++)
            {
                if (chars25[j] == chars25[i])
                {
                    found25 = true;
                    break;
                }
            }
            if (!found25)
            {
                chars25[write25] = chars25[i];
                write25++;
            }
        }
        for (int i = write25; i < chars25.Length; i++)
            chars25[i] = '_';
        Console.WriteLine(new string(chars25));

        // 26
        Console.Write("Введите предложение: ");
        string s26 = Console.ReadLine();
        char[] chars26 = s26.ToCharArray();
        int write26 = 0;
        for (int i = 0; i < chars26.Length; i++)
        {
            if (chars26[i] != 'о' || i % 2 == 1)
            {
                chars26[write26] = chars26[i];
                write26++;
            }
        }
        for (int i = write26; i < chars26.Length; i++)
            chars26[i] = '_';
        Console.WriteLine(new string(chars26));