What explains the increased concentration of in the ER?
A. Calcium levels in the blood or other body fluids are extremely low.
B. Calcium concentration is kept low in the cytoplasm because of its high usage level.
C. Calcium cannot enter the plasma membrane through ion channels.
D. Calcium ions are actively imported from the cytoplasm into the ER.
E. The CA+ ions are recycled from other molecules in the ER.
Answer: D