i want to save the checkbox selected value in database using gridview in asp.net

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C#

Tabel as follows (table Name Student_details)

Stud_id   Stud_name MobileNo

 1        Ram       99404542100
 2        Sam       97892112000
 3        Baskar    99454452121
 4        Suresh    97892512223
 5        Vikram    97905678242

I display the above Student_details into gridview.

My code as follows

public partial class _Default : System.Web.UI.Page 
{
    SqlConnection con = new SqlConnection("Server=(local);initial catalog=Master;Trusted_Connection=True");
    string str;
    SqlCommand cmd;

    protected void Page_Load(object sender, EventArgs e)
    {
        studentdetails();
    }

    private void studentdetails()
    {
        con.Open();
        SqlCommand cmd = new SqlCommand("select Stud_id,Stud_Name,Mobileno from Student_details order by Stud_Name", con);
        SqlDataAdapter adp = new SqlDataAdapter(cmd);
        DataSet ds = new DataSet();
        adp.Fill(ds);
        con.Close();
        gvstudentdetails.DataSource = ds;
        gvstudentdetails.DataBind();
    }

  protected void btn_checked_Click(object sender, EventArgs e)
    {

           foreach(GridViewRow gvrow in gvstudentdetails.Rows)
           {
              CheckBox chk = (CheckBox)gvstudentdetails.FindControl("chbox");
            if (chk != null & chk.Checked)
          {
con.Open();
                str = "insert into Student_details values('" + txt_studentid.Text.ToString().Trim() + "','" + txt_studname.Text.ToString().Trim() + "','" + txt_mobile.Text.ToString().Trim() + "')";
                cmd = new SqlCommand(str, con);
                cmd.ExecuteNonQuery();
                Response.Write("Record inserted");
                con.Close();

               }
          }
    }

when i run , in gridview 5 rows details are displayed in the gridview.
When i run i checked the first two rows values in database, that selected checkbox values is not saving in the database.

please help me what is the problem in my code.

Regards,
Narasiman P