You create a Microsoft ASP.NET application by using the Microsoft .NET Framework version 3.5. You create a Web form that contains the following code fragment：  
<asp：TextBox runat="server" ID="txtSearch" />  
<asp：Button runat="server" ID="btnSearch"
Text="Search" OnClick="btnSearch_Click" />  
<asp：GridView runat="server" ID="gridCities" />   
You write the following code segment in the code-behind file：  
01 protected void Page_Load（object sender， EventArgs e）  
02 {  
03   DataSet objDS = new DataSet（）；  
04   SqlDataAdapter objDA = new SqlDataAdapter（objCmd）；  
05   objDA.Fill（objDS）；  
06   gridCities.DataSource = objDs；  
07   gridCities.DataBind（）；  
08   Session["ds"] = objDS；  
09 }  
10 protected void btnSearch_Click（object sender， EventArgs e）  
11 {  
12 ► 
13 }   
You need to ensure that when the btnSearch Button control is clicked， the records in the gridCities GridView control are filtered by using the value of the txtSearch TextBox. Which code segment you should insert at line 12？（）

A.A
B.B
C.C
D.D