The answer is
C. The way the layer interacts with the layer below it.
Perhaps the answer is "physical characteristics". I am simply making an educated guess, however I do know early philosophers classified organisms into two groups, Plants and Animals, and identified them based on physical characteristics.
Biomolecules are organic compounds that are present as essential components of living things. There are four major biomolecules namely lipids, proteins, carbohydrates and nucleic acids. Proteins, lipids and glucose are usually used by organisms as energy sources as well as structural components among other functions in living organisms. Nucleic acids are important components that bring about heredity. In this case the appropriate molecule to be chosen by the long distance learner would be carbohydrate because it stores and may be used as a source of energy.
The given blank can be filled with the production of endorphins.
The endorphins are the hormones, which are responsible for the activation of the opiate receptors in the body. They are the natural pain killers. The activation of the opiate receptors causes the analgesic effects on the body.
These endorphins are secreted in the brain and the nervous system. The effect of the endorphins is similar to that of the morphine.
