This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Which one of the following is not true for an a-helix: 
Select and explain. 
A. It is one of the most impoant secondary structure, B. It has a net dipole moment, C. All hydrogen bonds are aligned in the same direction, D. Long stretches of left handed a-helices occur in proteins
D: Long stretches of left handed a-helices occur in proteins
Answer: D: Long stretches of left handed a-helices occur in proteins 
Explanation: D i.e. Long stretches of left handed a- helices occur in proteins