This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General anatomy, Subject name: Anatomy
Stage of 16 cells is 
Select and explain. 
A. Embryo, B. Morula, C. Zygote, D. Blastocyst
B: Morula
Answer: B: Morula 
Explanation: A morula is distinct from a blastocyst in that a morula (3-4 days post feilization) is a 16-cell mass in a spherical shape whereas a blastocyst (4-5 days post feilization) has a cavity inside the zona pellucida along with an inner cell mass