Which of the following statements about calcium ions in a typical cell is true?

• A. In a typical cell, calcium ions are far more abundant in the cytoplasm than in the blood and other extracellular fluid.
• B. In a typical cell, calcium ions are rapidly transported into the endoplasmic reticulum in response to G-protein-mediated signals.
• C. In a typical cell, calcium ions are not very widely used as second messengers.
• D. All of the listed responses are correct.
• E. In a typical cell, calcium ions are often concentrated within the endoplasmic reticulum.