The extracellular environment of the human body is typically abundant in sodium and calcium. In skeletal muscle cells, the sarcoplasmic reticulum is a specialized organelle that actively sequesters calcium from the cytosol.
It stockpiles the calcium until a motor neuron triggers its release through an action potential, which opens voltagegated calcium channels in the sarcoplasmic reticulum membrane. The calcium is necessary for the calciummediated functions of the protein troponin. In the presence of calcium, troponin removes tropomyosin from the myosin binding sites on actin filaments. This attachment is essential for sarcomere and muscle contraction.

If a muscle cell fails to contract, which of the following could be a reason?
(A) The cell is lacking tropomyosin.
(B) Too much calcium is in the sarcoplasmic reticulum.
(C) The cell is lacking troponin.
(D) Too many action potentials are reaching the cell.