Who put forward the sliding filament theory?

In which condition does relaxation occur in muscles?

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:

In striated muscle contraction:

$A-$ Visceral muscles do not show any striations and are smooth in appearance. Hence,they are known as non-striated muscles.
$R-$ Their activities are under the voluntary control of the nervous system.

Name the connective tissue sheath which surrounds the muscle bundles.