Harmonic mean temperature difference is given by_________________?

A. √(ΔT1 . ΔT2)
B. 2 (ΔT1 . ΔT2)/(ΔT1 + ΔT2)
C. 2 (ΔT1 . ΔT2)/(ΔT1 – ΔT2)
D. (ΔT1 – ΔT2)/(ΔT1 . ΔT2)