Mitral valve and tricuspid valve are attached to the papillary muscles by . . . . . . .

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:

Purkinje's fibres of the vertebrate heart are modified:

Where is the pacemaker located?

What causes the first heart sound?

The opening of the post caval vein into the right atrium is guarded by which of the following?