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: Anatomy
Neurulation is completed on day: 
Select and explain. 
A. 14, B. 18, C. 21, D. None of the above
D: None of the above
Answer: D: None of the above 
Explanation: Closure of the cranial neuropore occurs at approximately day 25 (18- to 20-somite stage), whereas the posterior neuropore closes at day 28 (25-somite stage). Neurulation is then complete, and the central nervous system is represented by a closed tubular structure with a narrow caudal poion, the spinal cord, and a much broader cephalic poion characterized by a number of dilations, the brain vesicles Ref: Langman's embryology 11th edition Chapter 5.