Given a scientific question, generate a correct answer to it.

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?
yolk sac

Liquid hcl can be used to do what to the ph of a swimming pool?
lower it

What spreads sti during sexual contact?
pathogens