Label the different components of the actin filament in the diagram given below.

The dark bands in a myofibril are due to the presence of:

Consider the following four statements $(a-d)$ and select the option which includes all the incorrect ones:
$(a)$ Skeletal muscle fibre is a syncytium as the sarcoplasm contains many nuclei.
$(b)$ In the centre of each $I$-band is an elastic fibre called $H$-line which bisects it.
$(c)$ In the resting state,a subunit of troponin masks the active binding sites for myosin on the actin filaments.
$(d)$ The contraction of a muscle fibre takes place by the sliding of the thick filaments over the thin filaments.

Which of the following statements is correct regarding muscle contraction?

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

$A$ collagenous connective tissue layer holds the muscle bundles together.