I believe this is a bit of a trick question, while you are right about both B and D, they both preexist they just develop into other structures. I believe the correct answer is A since the nerve cord is hollow not solid.
Sediment deposition and Animal waste
What would happen if M RNA were radio actively labeled instead of polypeptides is that instead of one seeing the DNA passing through all the molecular factories you will instead see just messenger RNA taking code and delivering it to Ribosome. This is mainly found in cytoplasm and Endoplasmic recticulums.
