The effectiveness of a heat exchanger in the $\varepsilon$-NTU method is defined as

1. $\frac{increase\ in temperature\ of\ the\ cold\ fluid}{decrease\ in\ temperature\ of\ the\ hot\ fluid}$
2. $\frac{actual\ exit\ temperature\ attained\ by\ the\ cold\ fluid}{maximum\ exit\ temperature\ attainable\ by the\ cold\ fluid}$
3. $\frac{actual\ exit\ temperature\ attained\ by\ the\ hot\ fluid}{minimum\ exit\ temperature\ attainable\ by\ the\ hot\ fluid}$
4. $\frac{actual\ heat\ transfer\ rate}{maximum\ possible\ heat\ transfer\ rate\ from\ hot\ fluid\ to\ cold\ fluid}$