下面那个代码段正确的增加了一个TextBox控件到一个名为form1的Web窗体上？（）

A、A．TextBox c = new TextBox(); c.ID = "txtUserName"; c.Visible = true;

B、B．TextBox c = new TextBox(); c.ID = "txtUserName"; c.Visible = true; c.Controls.Add(form1);

C、C．TextBox c = new TextBox(); c.ID = "txtUserName"; c.Visible = true; c.Add(form1);

D、D．TextBox c = new TextBox(); c.ID = "txtUserName"; c.Visible = true; form1.Controls.Add(c);