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.

Which of the following statements best describes the uptake of calcium by skeletal muscles and the sarcoplasmic reticulum?
(A) Calcium is passively taken up from the extracellular environment and the cytosol.
(B) Calcium is actively taken up from the extracellular environment and the cytosol.
(C) Calcium is passively taken up from the extracellular environment and actively taken up from the cytosol.
(D) Calcium is actively taken up from the extracellular environment and passively taken up from the cytosol.