On Average,how Many Amino Acids Engaged in Predominantly Hydrophobic $\alpha$ - Helices Does It Take to Cross the Hydrophobic Part

Multiple Choice

On average,how many amino acids engaged in predominantly hydrophobic $\alpha$ - helices does it take to cross the hydrophobic part of the membrane?

A) about 10 amino acids
B) about 20 amino acids
C) at least 40 amino acids
D) about 2 - 3 amino acids
E) None of these are correct.