In a healthy heart during exercise, cardiac output increases as heart rate increases up to about 120bpm, as stroke volume remains constant (CO = HR x SV). As heart rate goes further up, the cardiac output remains stable, and above 160bm it decreases. Why does it decrease?    A. shorter filling time decreases stroke volume  B. venous return decreases  C. valves cannot open and close fast enough  D. contractile power of heart muscle cells increases

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