You are implementing an ASP.NET application that includes a page named TestPage.aspx. TestPage.aspx uses a master page named TestMaster.master. You add the following code to the TestPage.aspx code-behind file to read a TestMaster.master public property named CityName. protected void Page_Load（object sender， EventArgs e） { string s = Master.CityName； }  You need to ensure that TestPage.aspx can access the CityName property. What should you do？（）