How both present and previous Textboes will be generated in my place holder

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ASP.NET

Hi
I have a dropdownlist ddlProduct Name and quantity text box. When I Select a Product Name and write the quantity in my textboxes and click the Button1 that number of textboxes are generating in my place holder according to quantity value. but when I am again go to choose the another product and quantity and click the Button 1 then the current quantity textboxes are generating and not showing the previous selected textboxes. I need to generate the textboes of present product and previous. Both should appear in my place holder.How will i Do it. Kindly Help.

protected void Button1_Click(object sender, EventArgs e)
    {

        {
            if (txtQuantity.Text != "")
            {
                for (int i = 0; i < Convert.ToInt16(txtQuantity.Text); i++)
                {
                  
                    
                        Label la = new Label();
                        la.Text = ddlProductName.SelectedItem.Value + i.ToString();
                        la.ID = "abc" + i.ToString();

                        PlaceHolder1.Controls.Add(la);

                        //la = FindControl(String.Format(i)) as Label;




                        TextBox txtbox = new TextBox();
                        txtbox.Text = "Textbox - " + i.ToString();


                        PlaceHolder1.Controls.Add(new LiteralControl("<input id='txt' name='Textbox" + i + "'type='text'  />"));
                        PlaceHolder1.Controls.Add(new LiteralControl("<br />"));

                    }
                }
            }
        
    }