using System;
using System.Collections.Generic;

namespace ConsoleApplication62
{
    class Program
    {
        static void Main(string[] args)
        {
            var a = new string[]{"a1", "a2", "a3", "a4", "a5"};
            var b = new string[]{"b1", "b2"};
            foreach (var s in BuildItems(a, b))
                Console.WriteLine(s);
            Console.ReadLine();
        }

        static IEnumerable<string> BuildItems(string[] a, string[] b)
        {
            long maxA = (long)Math.Pow(2, a.Length);
            for (long i=1; i<maxA; i++)
            {
                long mask = i;
                string resultA = "";
                for(int k=0;k<a.Length;k++)
                {
                    if((mask & 1) == 1)
                        resultA += a[k];
                    mask = mask >> 1;
                }

                for (int j = 0; j < b.Length; j++)
                    yield return resultA + b[j];
            }
        }
    }
}