$A$: Isovolumetric systole of a normal cardiac cycle is responsible for the opening of semilunar valves causing the blood flow into aortic aorta and pulmonary aorta.
$R$: During isovolumetric systole,intraventricular pressure increases as semilunar and $AV$ valves are closed and ventricles are contracting.

• A
  Assertion and Reason both are correct and also correct explanation.
• B
  Assertion and Reason both are correct but not explanation of assertion.
• C
  Assertion is correct,but Reason is incorrect.
• D
  Both Assertion and Reason are incorrect.