Answer:
they compare fossils to other fossils to see if skeleton changes are observed in a species over time it indicates that there is genetic change going on within the species
Im not sure but if you think of it, the one that you didn't put any salt is a independent variable because it doesn't really rely on anything it doesn't have salt. The Dependent variable would be the 2nd and the 3rd cup because it doesn't have much salt but the 4th cup has lots of salt so its basically controlled. Hope it helps !
It is only active at a pH of around 2 because that is around the same pH as the gastric juices in the stomach which is where pepsin is used to break down proteins.
From what I understood, the answer is 2a (: