This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: parasitology, Subject name: Microbiology
Which of the following show regressive metamorphosis - 
Select and explain. 
A. Hydatid cyst, B. Cysticercoid, C. Cysticercus bovis, D. Cysticercus cellulosae
A: Hydatid cyst
Answer: A: Hydatid cyst 
Explanation: Retrogressive metamorphosis is when an organism changes from a more advanced form to a primitive one.the ruptured hydatid cyst releases protoscolices into tissues which do not develop into adult forms according to law of migration of cestodes.... but they form a new hydatid tissue against the law.... this is called regressive metamorphosis.