问题：

[单选]

You create a Microsoft ASP.NET application by using the Microsoft .NET Framework version 3.5. You create two user controls named UserCtrlA.ascx and UserCtrlB.ascx. The user controls postback to the server. 
You create a new Web page that has the following ASPX code：  
<asp：CheckBox ID="Chk" runat="server"    
oncheckedchanged="Chk_CheckedChanged"   
AutoPostBack="true" />  
<asp：PlaceHolder ID="PlHolder" runat="server"></asp：PlaceHolder>  
To dynamically create the user controls， you write the following code segment for the Web page：  public void LoadControls（） 
{ 
if （ViewState["CtrlA"] != null） 
{ 
Control c； 
if （（bool）ViewState["CtrlA"] == true） 
{ 
c = LoadControl（"UserCtrlA.ascx"）； 
} else { 
c = LoadControl（"UserCtrlB.ascx"）； 
}  
c.ID = "Ctrl"； 
PlHolder.Controls.Add（c）； 
} 
}  
protected void Chk_CheckedChanged（object sender， EventArgs e） 
{ 
ViewState["CtrlA"] = Chk.Checked； PlHolder.Controls.Clear（）； 
LoadControls（）； 
}  
You need to ensure that the user control that is displayed meets the following requirements： It is recreated during postback It retains its state.  
Which method should you add to the Web page？（）

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