Which Example Best Describes Both an Elemental Pool Where an Element

Multiple Choice

Which example best describes both an elemental pool where an element accumulates and an elemental sink where it is removed from circulation and locked up for long periods?

A) Calcium in bones,which remains fairly stable throughout an animal's life
B) Nitrogen that is made available to plants from their bacterial endosymbionts
C) Fossil fuel deposits in which carbon is stored for hundreds of millions of years
D) Atmospheric oxygen levels,which are maintained stable by photosynthesis
E) Sulfur compounds,which react with water to produce acid precipitation