Heart murmurs are indicative of a defect in which of the following?

The first heart sound is associated with which of the following?

Following are the stages of the pathway for the conduction of an action potential through the heart:
$A.$ $AV$ bundle
$B.$ Purkinje fibres
$C.$ $AV$ node
$D.$ Bundle branches
$E.$ $SA$ node
Choose the correct sequence of the pathway from the options given below:

The circulation of blood was discovered by .....

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

Assertion : $Lub$ is a heart sound which is produced during each cardiac cycle.
Reason : It is associated with the closure of the tricuspid and bicuspid valves.