Which of the following structures contract and relax rhythmically to produce movement?

Cardiac muscle fibres are

$A : Ca^{2+}$ plays an important role in muscle contraction.
$R : Ca^{2+}$ combines with the troponin chain,displacing tropomyosin and allowing the myosin head to combine with actin to form an actomyosin complex.

During muscle contraction,which of the following statements is false?

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

The single-unit smooth muscles are: