Which of the following substances can cause vasodilation?

Statement-$I$ :- The body has the ability to alter the stroke volume as well as heart rate and thereby the cardiac output.
Statement-$II$ :- During each cardiac cycle two prominent sounds are produced which can not be easily heard through a stethoscope.

Newborn babies generally do not shiver despite low temperatures because of:

Analogy type questions:
$(1)$ Fish : Single circulation :: Amphibians : ...........
$(2)$ Between right atrium and right ventricle : Tricuspid valve :: Between left atrium and left ventricle : .............

Select the correct option regarding the circulatory system.

Which of the following is an incorrect combination?