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
Implantation occurs at the stage of 
Select and explain. 
A. Zygote, B. Morula, C. Blastocyst, D. Primary villi
C: Blastocyst
Answer: C: Blastocyst 
Explanation: C i.e. Blastocyst- Barrier is provided by fusion / feilin (not Ach)Q. Fusion reduce the membrane potential of ovum that prevents polyspermy by not allowing the others sperms to enter.- Morula is a 16 cell stage, surrounded by zona pellucida. Barrier is provided by fusiotVfetilin.QImplantation occurs at the stage of blasto cystQ.