The mitral valve is supported by which of the following structures?

Which structure initiates the heartbeat?

During human fetal development, the pulmonary artery is connected to the aorta by a narrow duct called the ductus arteriosus. What is this structure called after it closes and becomes a fibrous remnant in an adult?

When is the '$DUB$' sound of the heart heard?

Given below are two statements.
Statement $I$ - The heart sound $Dub$ is heard during ventricular diastole.
Statement $II$ - The heart sound $Lubb$ is heard during atrial systole.
In the light of above statements,select the correct option given below:

What is the maximum efficiency of the heart?