命名空间“System.Web”中不存在类型或命名空间名称“Script”(是缺少程序集引用吗?)

using System;
using System.Collections;
using System.Web;
using System.Web.Services;
using System.Web.Services.Protocols;

/// <summary>
///自动完成
/// </summary>
[WebService(Namespace = "http://)]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.Web.Script.Services.ScriptService]
public class PhraseService : System.Web.Services.WebService {

    public PhraseService () {

        //如果使用设计的组件，请取消注释以下行
        //InitializeComponent();
    }

    [WebMethod]
    public string[] Get(string prefixText, int count)
    {
        phrase p = new phrase();
        System.Data.DataTable dt = p.GetPhrase2(prefixText, count.ToString()).Tables[0];
        string[] result = new string[dt.Rows.Count];
        for (int i = 0; i < dt.Rows.Count; i++)
        {
            result.SetValue(dt.Rows[i][0].ToString(), i);
        }
        return result;
    }
   
}

错误提示:错误    1    命名空间“System.Web”中不存在类型或命名空间名称“Script”(是缺少程序集引用吗?)    F:\Inetpub\wwwroot\website\google\App_Code\PhraseService.cs    11    13    F:\...\google\