Ratio of tube length to shell diameter for a shell and tube heat exchanger is________________?

A. 8 : 1 to 12 : 1 for both liquid-liquid and gas-gas heat exchangers
B. 4 : 1 to 8 : 1 for liquid-liquid exchanger
C. < 4 : 1 for gas-gas exchangers
D. Both B. & C.