In the myofibrils of muscle fibre:

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.

What is the dark line lying in the middle of the $H$-zone called?

The major protein in the thick filament of skeletal muscle fibre is

The functional unit of contraction is a

In a skeletal muscle fiber,what is the $H$-zone?