Getting error like are you missing an assembly reference in my program

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Speech.Recognition;
using System.Threading;

namespace SpeechRecogTest
{
    public partial class Form1 : Form
    {
        SpeechRecognitionEngine sr = new SpeechRecognitionEngine(); 

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            //Create grammar
            Choices words = new Choices(); 
            words.Add("Hi");
            words.Add("No");
            words.Add("Yes");

            Grammar wordsList = new Grammar(new GrammarBuilder(words));

            wordsList.SpeechRecognized += new EventHandler<SpeechRecognizedEventArgs>(rec_SpeechRecognized);
            sr.LoadGrammar(wordsList);

        }

        void rec_SpeechRecognized(object sender, RecognitionEventArgs e)
        {
            MessageBox.Show(e.Result.Text);
        }
    }
}