To Calculate the Mean Sac Diameter of a Gestational Sac $\times$

To calculate the mean sac diameter of a gestational sac, which one of the following formulas should be used?

A) Length $\times$ width $\times$ height
B) Length + width + height
C) (Length $\times$ width $\times$ height) ÷ 3
D) (Length + width + height) ÷ 3