I think it is C. <span>Obstacles make us stronger, because a theme is the central idea of the story but D also kinda sounds like a theme.
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Answer:
the answer at b says that the structure of the heart at animal x is straight and inbetween the Lungs but at animal y the structure of the heart is slightly tilted to the left
and the answer at c the most efficient blood circulation is at y because the blood is able to flood accordingly and it it well transported in the aorta
