The correct answer is - oxygen.
The phytoplankton and the microscopic marine plants are using the process of photosynthesis in order to produce energy for them. In order for this process to take place, two things are needed, sunlight and carbon dioxide. The consumed carbon dioxide is not totally used, but there's some waste from it. The waste is actually the oxygen. The oxygen is released and it gets back into the atmosphere. This gas is crucial for the survival of the living organisms on the planet, especially the animals, so it is extremely import that the environment of the phytoplankton and the microscopic marine plants is not destroyed, as they are the ones that release the biggest amount of oxygen into the atmosphere.
Peripheral nervous system
The answer is A. Can mate with each other and produce fertile offspring.
