A) Blood flows from X to Y.
B) Muscles in the wall relax and close the valve, preventing backflow.
C) The elastic wall causes the valve to close between heart beats.
D) The valve is forced open when the blood pressure at Y is greater than at X.
Correct Answer: D
Solution :Blood moves from high pressure area to low pressure area. So in the diagram, the valve gets open forcefully when the blood pressure at 'Y is greater than at 'X.
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