In the resting state,a subunit of troponin masks the active binding sites for .......... on the .......... filaments.

Given below are two statements $:-$
Statement-$I$ :- Each organised skeletal muscle in human body is made of a number of fascicles,held together by a common connective tissue layer called fascia.
Statement-$II$ :- The portion of the myofibril between two successive '$Z$' lines is considered as the functional unit of muscle contraction,known as a sarcomere.
In the light of the above statements,choose the correct answer from the options given below $:-$

Describe the important steps in muscle contraction.

Name the ions responsible for unmasking the active sites on myosin for cross-bridge formation during muscle contraction.

Which one of the following takes place in a myofibril when skeletal muscle contracts?
$A$-band (Dark band) | $I$-band (Light band) | $H$-zone

Contraction of muscles is the best example of conversion of cellular energy into