During muscle contraction in a skeletal muscle fibre,$Ca^{2+}$ combines with

The main source of energy during muscle contraction is

Which is the smallest muscle in the human body?

The muscles immune to fatigue are

Muscles are red because of the presence of

Which of the following statements about the molecular arrangement of actin and myosin are correct?
$I.$ Each actin (thin filament) is made up of $2$ $F$ (filamentous) actins.
$II.$ $F$-actin is the polymer of $G$ (globular) actin.
$III.$ $2$ $F$-actins are twisted into a helix.
$IV.$ Two strands of tropomyosin (protein) lie in the grooves of $F$-actin.
The correct option is: