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Doctors use stethoscope to hear the sound produced during each cardiac cycle. The second sound is heard when:

  1. A
    AV node receives signal from SA node
  2. B
    AV valves open up
  3. C
    Ventricular walls vibrate due to gushing of blood from atria
  4. D
    Semilunar valves close down after the blood flows into vessels from ventricles

Solution & Step-by-step Explanation

The first heart sound ('lub') is due to the closure of AV valves. The second heart sound ('dup') is produced by the closure of the semilunar valves at the beginning of ventricular diastole.

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Doctors use stethoscope to hear the sound produced during each cardiac cycle. The second sound is heard when:
A
AV node receives signal from SA node
B
AV valves open up
C
Ventricular walls vibrate due to gushing of blood from atria
D
Semilunar valves close down after the blood flows into vessels from ventricles

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