Final answer:
During isovolumic contraction in the cardiac cycle, the ventricle muscles contract causing pressure to rise, but there's no change in the volume of the blood in the ventricles as the semilunar valves have not yet opened, so three choices are true. The correct option is 1.
Step-by-step explanation:
In addressing which event is happening at a specific point of the cardiac cycle, we must consider the description provided. Initially, the ventricle muscles contract and the pressure rises within the ventricles. However, this pressure is not yet sufficient to open the semilunar valves to allow for ejection of blood. Consequently, the tricuspid and mitral valves close due to the blood attempting to flow back into the relaxed atria, which are in diastole.
There is no change in the blood volume within the ventricles at this stage, hence this phase is known as isovolumic contraction or isovolumetric contraction. This description aligns with option 2) 'There is no change in volume' and 4) 'Pressure is either rapidly increasing or rapidly decreasing in the ventricles.' Since a set of valves (tricuspid and mitral) has just closed and the semilunar valves are about to open, option 5) 'A set of valves is getting ready to open' is also correct.
Therefore, the correct option in this scenario, considering the three statements that are true, is 1) 'Three choices are true'.