how to identify which dropdownlist is selected in asp.net

Question

Design as follows
 
GRADING: EXCELLENT\VERY GOOD\GOOD\FAIR\POOR
 
To what extent equipment efficiency is user friendly Dropdownlist1
Value for course Fees Dropdownlist2
Decision to choosen HIMT Dropdownlist3
 

In the above dropdownlist when i select the Poor from any of thedropdownlist, popup screen will open in that one textbox is there reason for poor will type.
 
Then i have one ok button and click that button.
 
when i select the any dropdownlist from the above, commonly one popup will be open if we select the above dropdownlist in that popup screen one textbox is there reason for pooor will type.
 
my question is how do you know which dropdownlist is selected and give the reasonf for poor.
 
for that how can i do in asp.net.
 
Regards,
narasiman P