Match the following columns:

Column-$I$	Column-$II$
$P$. Skeletal muscle	$I$. Biceps
$Q$. Smooth muscle	$II$. Iris
$R$. Cardiac muscle	$III$. Heart

$A$: Application of stimuli repeatedly just at the start of relaxation of a muscle fibre shows an increase in the extent of contraction initially and results in treppe or staircase phenomenon.
$R$: It can be attributed to the summation effect of the sub-threshold stimuli being given again and again.

Muscle fatigue occurs due to the accumulation of:

The storehouse of calcium ions in the muscle fibre is

What are $P, Q,$ and $R$ in the given diagram?
$P \quad\quad Q \quad \quad R$

Which of the following muscles gets fatigued very early?