LMTD for evaporators & condensers for a given terminal parameters & set of conditions for counter$-$flow is equal to that for parallel flow. In such heat exchangers, with one of the fluids condensing or evaporating, the surface area required is the least in the $\_\_\_\_\_\_\_\_\_$ flow.

Parallel

Mixed

Counter flow

Same in either $\text{'}$a$\text{'},$ b$\text{'}$ or $\text{'}$c$\text{'}$