in dropdown particular item select want to show Modalpopup Extender

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C#

Design as follows

Course
For the below 3 dropdownlist (Exellent\Good\Poor) are there in the dropdownlist.
To What extent was this course useful Dropdownlist1(Exellent\Good\Poor)
To What extent equipment efficiency is user friendly Dropdownlist2(Exellent\Good\Poor)
Decision to have chosen our institute Dropdownlist3(Exellent\Good\Poor)

When i choose the Poor from the dropdownlist i want to show the Modalpopup Extender.

My code as follows

protected void Dropdownlist1_SelectedIndexChanged(object sender, EventArgs e)
{
     if (Dropdownlist1.Text.ToString() == "Poor")
     {
         ModalPopupExtender1.Show();
         Pnlnegativefeedback.Visible = true;
         return;
     }
}

But when i run and click the Dropdownlist1 Modalpopup Extender is open. But i want the Modalpopup Extender to open only when i select the Poor from the Dropdownlist1.

from my above code what is the problem.

please help me.

Regards,
Narasiman P.