Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Cells of different tissues essentially require $CO_2$ to generate energy and perform metabolic functions.
Reason (R) : Deficiency of oxygen or hypoxia results in failure to carry metabolic functions.
In the light of the above statements, choose the most appropriate answer from the options given below :

Depending upon the antigen how much lag period is required for memory B cell after antigen administration.

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : The conduction in the Atrio-ventricular (AV) node is slow i.e. 0.05 m/sec, there is a delay of approx. 0.1 second before excitation spreads to the ventricles.
Reason (R) : AV node delay enables the atria to empty the blood present within them into the ventricles before these contract.
In the light of the above statements, choose the most appropriate answer from the options given below :