Select the correct option:
$(1)$ $AB / O$ blood group is the universal recipient.
$(2)$ Mitral valve / Semilunar valve is located between the atrium and the ventricle.

Choose the correct option:
$(1)$ Depolarization of the ventricles induces the $QRS$ / $T$ waves.
$(2)$ Incomplete double circulation is seen in reptiles / birds.

Assertion $A$: In an open circulatory system, the blood volume is high, but the blood pressure is low and irregular.
Reason $R$: In a closed circulatory system, the blood volume is limited, and the blood pressure is high and regular.
Which option is correct for Assertion $A$ and Reason $R$?

Hypokalaemia means

Match the following:
Types of leucocytes | Function
$A.$ Neutrophils | $1.$ Heparin and histamine secretion
$B.$ Basophils | $2.$ Antibodies formation
$C.$ Acidophils | $3.$ Scavenger
$D.$ Monocytes | $4.$ Phagocytes
$E.$ Lymphocytes | $5.$ Antiallergic and healing of wounds
The correct pairing sequence is:

$A$: Elevated levels of $Na^+$ decrease the heart rate and contractility.
$R$: Elevated $Na^+$ levels increase the excitability of the $SA$ node.