Can Anyone give me solution

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ASP.NET

I have created a one form which consits of various fields.
this fields data i have succesfully saved in the database.
Now i have created one more form which consits of one drop down list.
i want drop down list should contain a primary key value,and after selecting a particular value value from dropdown list the whole details of that associated primary key should be display in the gridview.

This is what i have done in the first page.
In the following code stdid is a Primary key of the table.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Data;
public partial class FillInformation : System.Web.UI.Page
{
    SqlConnection con;
        
    protected void Page_Load(object sender, EventArgs e)
    {
       

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        SqlConnection con= new SqlConnection("server=SACHIN-5DDBFA03\\SACHIN;uid=sa;password=india;database=master");
        con.Open();
        SqlCommand cmd = new SqlCommand("insert into Students_Details values(@stdid,@sfname,@slname,@Maths,@Science,@SocialScience,@Address)",con);
        cmd.Parameters.Add("@stdid",txtrollnumber.Text);
        cmd.Parameters.Add("@sfname",txtfname.Text);
        cmd.Parameters.Add("@slname",txtlname.Text);
        cmd.Parameters.Add("@Maths", txtmath.Text);
        cmd.Parameters.Add("@Science", txtscience.Text);
        cmd.Parameters.Add("@SocialScience", txtsoscience.Text);
        cmd.Parameters.Add("@Address",txtaddress.Text);

        cmd.ExecuteNonQuery();
        con.Close();

        Response.Write("One Record Inserted Succesfully");
        

    }
    protected void next_Click(object sender, EventArgs e)
    {
        Response.Redirect("Students_Details.aspx");
    }
}

can any 1 give coding for the second page.
Please give me easy solution as much as possible.