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

i am using this code in .net framework 3.5 , for creating a grammar using visual studio 2010 professional in win 7 but i am getting errors like
The type or namespace name 'Speech' does not exist in system (are you missing a using directive or an assembly reference.

The type or namespace name 'SpeechRecognitionType' could not be found (are you missing a using directive or an assembly reference,

similarly others like type linq does not exist are you an missing an assembly refernce.

`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);
    }
}

}`