How do I change the variable in only one inctance?

I have 5 cities with 5 rechargeable shields powered by each city so when city #1 is destroyed how do I tell shield 1 and only shield 1 to change its recharge variable from 1 to 0 thus turning it off.

Give the shield a second variable , call it city , number it 1 to 5 , when city1 is destroyed tell shield with variable city 1 to turn off .

Thank and that's very similar to what I ended up doing though my method may have been more complicated.

I assigned each city and shield a variable I named Ego then at the start of layout I did an event that made Ego = IID so the then I could basically call them by their ID which I couldn't figure out how to do before but now I was Just calling them by their Ego