using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var chs = new char[] {'a', 'b'};

        var test = new string[] {"hallob", "hello", "hallo", "abq"};

        var filtered = new List<string>(test.Length);

        foreach (var t in test) {

            var viable = true;

            foreach (var ch in chs) {

                if (!t.Contains(ch.ToString())) {

                    viable = false;

                }

            }

            if (viable) filtered.Add(t);

        }

        foreach (var f in filtered) {

            Console.WriteLine(f);

        }

    }

}