setting multiple conditions to a single output value of a drop down list enabling certain text boxes accordingly

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C#

ASP.NET

whenever i select student the roll number text box gets visible,but the glitch is when i go back and change the drop down list value to something else again,both the roll number and employee id text box become visible.can somebody please help me out.i'll be ever so greatfull.

protected void DropDownListdes_SelectedIndexChanged(object sender, EventArgs e)
   {
       Label1.Text = DropDownListdes.SelectedItem.ToString();
       if (Label1.Text == "student")
       {
           Labelyr.Visible = true;
           DropDownListyr.Visible = true;
           Labelroll.Visible = true;
           collegerollno.Visible = true;

       }
       else if (Label1.Text == "principal")
       {

           DropDownListdept.Items[3].Enabled = true;
           LabelEID.Visible = true;
           TextBoxEID.Visible = true;
       }
       else
       {
           LabelEID.Visible = true;
           TextBoxEID.Visible = true;
       }