Cognos Event studio: issue with event condition

[Jithu]

Guys,

I am new to cognos and cognos event studio. I need to create an event where is the event condition is based on a drop down field . The drop down field has 4 values say [a,b,c,d]. I need the event to be triggered exactly when value 'C' is changed to 'd' and not when 'a' changed to 'd' or when 'c' changed to 'd'. Kindly suggest how to keep the previous value in memory and compare with current value.

Thanks!!