The contractile unit of a muscle fiber is known as:

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.

$A$: Latent period is the interval between the application of an appropriate stimulus and the initiation of contraction.
$R$: Latent period is minimum in cardiac muscle fibres.

The single-unit smooth muscles are:

Assertion: Fatigue is the inability of a muscle to relax.
Reason: It is due to lactic acid accumulation by repeated contractions.

Which of the following activities occurs at the site labeled $P$ in the given diagram of a myosin monomer?