What happens when a sub-threshold stimulus is applied to a muscle?

$ATP_{ase}$ activity is found in:

The mineral element required for blood clotting is also essential for which other process?

Select the correct option for skeletal muscle.
$(I)$ In human body,it is made of a number of muscle bundles.
$(II)$ Muscle bundles contain a number of muscle fibers and each fiber is lined by the sarcolemma.
$(III)$ In a muscle fiber,the sarcoplasm contains only one nucleus.
$(IV)$ In a muscle fiber,the sarcoplasmic reticulum is the storehouse of calcium ions.

Actin filament is made up of

Rahul exercises regularly by visiting a gymnasium. Of late,he is gaining weight. What could be the reason? Choose the correct answer and elaborate.
$A$. Rahul has gained weight due to accumulation of fats in the body.
$B$. Rahul has gained weight due to increased muscle mass and loss of fat.
$C$. Rahul has gained weight because his muscle shape has improved.
$D$. Rahul has gained weight because he is accumulating water in the body.