I believe the medulla is the answer.
Answer:
the description of a mineral species usually includes its common physical properties such as habit, hardness, lustre, diaphaneity, colour, streak, tenacity, cleavage, fracture, parting, specific gravity, magnetism, fluorescence, radioactivity, as well as its taste or smell and its reaction to acid.
The answer is b (bladder)
An acid is a proton donor
The change is that the piston gets hotter? (Theres more heat in it)
