Answer:
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Marine biologists study changes in the ocean. so they will look at the chemical composition of deep waters and see how consecrated the oxygen is. They will also study the movements of the plates because when planted move it usually makes new food for deep life
they will also check animal concentrations in certain areas.
Another thing they do is track animal movements. this could be by putting a tracker on a whale or a shark or anything and watching its depths it goes or watching the distance it goes in a day.
Answer:
The blood travels back to the heart to get oxygen.
D is the best answer
(I am just using my common sense)
Hope I helped :)
