For annular flow of a fluid, the ratio of the equivalent diameter for pressure drop calculation to the equivalent diameter for heat transfer calculation is_________________?

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For annular flow of a fluid, the ratio of the equivalent diameter for pressure drop calculation to the equivalent diameter for heat transfer calculation is_________________?

A. d1/(d1 + d2)
B. d2/(d1 + d2)
C. (d1 + d2)/d1
D. (d1 + d2)/d2