instruction:
Given a scientific question, generate a correct answer to it.
question:
On the ventral side of the embryonic disc, opposite the amnion, cells in the lower layer of the embryonic disk (the hypoblast) extend into the blastocyst cavity and form this?
answer:
yolk sac

question:
What living things are considered photoautotrophs because they use sunlight and carbon from carbon dioxide to synthesize chemical energy in the form of carbohydrates?
answer:
plants

question:
Protecting the surface of metal prevents what?
answer:
corrosion