[单选题]
The diagram below shows the structure
.nm/ fk xw/kx. of
e :.oe0upt5jydq 4.l lr p08dgthe human heart.
Which of the following statement(s) is/are true?
I. When the walls of the atria contract, blood flows from the atria to the ventricles because the atrioventricular valves are open
II. When the walls of the ventricles contract, the increase in pressure causes the atrioventricular valves to close which prevents the backflow of blood from the ventricles to the atria.
III. When the ventricles stop contracting, the pressure falls and the semilunar valves close preventing backflow from the arteries to the ventricles.
