using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static List<string> list = new List<string>{"abc", "xyza", "1234", "54625g67a78a", "|436afda", "oh, shit"};

    public static void Main()

    {

        foreach(var itemA in A())

            Console.Write(itemA);

        Console.WriteLine();

        foreach(var itemB in B())

            Console.Write(itemB);

    }

    public static IEnumerable<char> A(){

        foreach (var a in list.Where(s => s.Contains("a")))

        {

            foreach (var b in a)

            {

                yield return b;

            }

        }

    }

    public static IEnumerable<char> B()

    {

        return list.Where(s => s.Contains("a"))

            .SelectMany(a => a);

    }

}