How to pass Parameter in the query according to the dropdown.

this is .aspx file
--------------------------
<table>
<tr>
<td>

<select id="drpSelect1" runat="server" name="D1">
<option>-- Select --</option>
<option value="BDate">Birth Date</option>
<option value="Mob_No">Mobile no</option>
<option value="F_MobNo">Father Mobile no</option>
<option value="Cat_Id">Catagory</option>
<option value="Str_Id">Stream</option>
</select>
</td>
<td>
<asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" />
</td>
</tr>
<tr>
<asp:GridView ID="GridView1" runat="server" BorderWidth="1px" CellPadding="4" ForeColor="Black"
GridLines="Horizontal" BackColor="White" BorderColor="Black"
BorderStyle="None" AllowPaging="True" PageSize="20">
<footerstyle backcolor="#CCCC99" forecolor="Black">
<PagerStyle BackColor="White" ForeColor="Black" HorizontalAlign="Right" />
<SelectedRowStyle BackColor="#CC3333" BorderColor="Black" Font-Bold="True" ForeColor="White" />
<HeaderStyle BackColor="#333333" Font-Bold="True" ForeColor="White" />

</tr>
</table>



this is .aspx.cs file
------------------------
public partial class Admin_Default : System.Web.UI.Page
{
public CommanFunctions co = new CommanFunctions();
public SqlCommand cmd = null;
protected void Button1_Click(object sender, EventArgs e)
{
GridView1.Visible = true;
FillGrid();
}
void FillGrid()
{
co.con.Open();
string str1 = "SELECT SurName, Name, FName, BDate FROM Student_Details";
if (drpSelect1.Value == "BDate")
cmd = new SqlCommand(str1, co.con);
DataSet ds1 = new DataSet();
cmd.ExecuteNonQuery();
co.adpt = new SqlDataAdapter(cmd);
co.adpt.Fill(ds1);

GridView1.DataSource = ds1;
GridView1.DataBind();
co.con.Close();
}
}

and i want "Bdate" pass into the select Query. which is selected from the dropdown menu.

string str1 = "SELECT SurName, Name, FName, BDate FROM Student_Details";