Paper 1 inquiry

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The answer is (A- W and X) because as we know W and X are atria. Y and Z are ventricles.
When W and X contract, they pump blood to the ventricles and this is done at the same time.
This explains the term : "double circulation" in which ox. blood from the lungs and deox. blood from the body enter the blood at THE SAME TIME causing the atria to contract to pump this blood to the ventricles.
If you forgot this, look at the other choices, they all dont make sense ;)
 
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