Question

which of the following is the main reservoir of bicarbonate ions?
sedimentary rocks
fossil fuels
water
volcanoes

Explanation:

Bicarbonate ions ($HCO_3^-$) are abundant in water. In the Earth - water system, carbon dioxide dissolves in water and forms carbonic acid which dissociates to bicarbonate ions. Sedimentary rocks mainly contain carbonate minerals rather than bicarbonate ions in large amounts. Fossil fuels are mainly composed of hydrocarbons and do not serve as a main reservoir for bicarbonate ions. Volcanoes release carbon - containing gases like carbon dioxide but are not the main reservoir of bicarbonate ions.

Answer:

C. Water