During muscle contraction,which of the following events occurs?

The storehouse of calcium ions in the muscle fibre is

$ATPase$ enzyme needed for muscle contraction is located in

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.

Flexion of the thigh,knee,and their rotation is facilitated by:

Contraction of muscle fibre takes place by the sliding of $:-$