【简答题】
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);