c# – 在Web Api控制器中无法识别Json方法

using sport.BLL.Abstract;
using sport.BLL.Concrete;
using sport.DAL.Entities;
using sport.webApi.Models;
using AutoMapper;
using Microsoft.AspNet.Identity.EntityFramework;
using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.AspNet.Identity;
using Microsoft.AspNet.Identity.Owin;  
using Microsoft.Owin.Security;
using System.Net;
using System.Net.Http;
using System.Web.Http;
using System.Web;
using System.Web.WebPages.Html;

namespace sport.webApi.Controllers 
{
    public class AccountManageController : ApiController
    {
        [HttpPost]
        public System.Web.Mvc.ActionResult CreateAccount(CollaborateurModel item)
        {
            var user = new ApplicationUser { UserName = item.Username,Email = item.Email };
            var result = UserManager.CreateAsync(user,item.Password);
            if (result.Result.Succeeded)
            {

                var currentUser = UserManager.FindByName(item.Username);
                var roleresult = UserManager.AddToRole(currentUser.Id,item.Role);
                ajt_collaborator entity = Mapper.Map<CollaborateurModel,ajt_collaborator>(item);
                entity.id_user_fk = currentUser.Id;
                entity.is_deleted = false; 
                repo.CreateCollaborator(entity); 
                var response = new { Success = true }; 
                return  Json(response);


            }
            else
            {
                var errorResponse = new { Success = false,ErrorMessage = "error" };
                return  Json(errorResponse);
            }

        }
    }
}

我在这行中遇到错误：

返回Json(回应);

Json方法无法识别!!!当我用Google搜索时,我得到了这个link,表明Json方法包含在System.Web.Mvc中.即使我尝试导入此命名空间,我得到相同的错误？

>这个错误的原因是什么？
>我该如何解决？