This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Pathology
Upon contact between the sperm head and the zona pellucida, penetration of the sperm into the egg is allowed because of 
Select and explain. 
A. The acrosome reaction, B. The zona reaction, C. The perivitelline space, D. Pronuclei formation
A: The acrosome reaction
Answer: A: The acrosome reaction 
Explanation: .