The type or namespace name Linq does not exists in the namespace system

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;


namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            string path = "D:/Project/";
            //string[] fileNames = { "A_L_PRTD_432124.txt", "A_L_PRTD_021345.txt" };
            string fileNameStartsWIth = "A_L_PRTD_";
            string[] fileNames = Directory
                 .GetFiles(path, "*.txt", SearchOption.AllDirectories)
                 .Select(f => Path.GetFileName(f)).Where(k => k.StartsWith(fileNameStartsWIth)).ToArray();

            string finalFile = "L_PRTD_" + DateTime.Now.ToString("ddMMyyyy") + ".txt";

          List<string> lstLines = new List<string>();
            foreach (string file in fileNames)
            {
                string[] content = File.ReadAllLines(path + file).Select(k => k.Trim()).Distinct().ToArray();
                lstLines.AddRange(content);
            }
            var data = lstLines.Distinct();
            File.AppendAllLines(path + finalFile, data);


        }
    }
}