GridView, popup when second last row changes?

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C#

C#4.0

Hello Everyone,
I have this gridview on my web page

A B C D
1 2 3 4
8 34 95 100
10 8 95 60
25 4 95
70 32 78 96
34 88 78 200
column "A" is a linkbutton
I want to show the user a pop up window if the second last row changes so for e.g if the user clicks on 8 then I will look for value in column "C" and the value in column C is 95 so I won't show the user a pop window if he clicks on 8, 10 and 25, but i will show him a pop up windows if he clciks on 70, 34, and 1.
Similarly if the user clicks on 70 then I won't show the user a pop up window if he clicks on 34, but if the user clicks on 1, 8, 10, 25 then I will show him a pop window saying that "The second last row changed"
How can I implement this logic in an elegant way.