[Solved] A pH value of ____ has the highest concentration of hydrogen ions. A)1 B)3 C)5 D)7 E)9 Carbonate Rock: Sedimentary rocks, such as limestone, that are composed primarily of carbonate minerals. Microscopic Organisms: Tiny life forms, often unicellular, that are not visible to the naked eye and require a microscope to be observed. Mass Extinction: Mass Extinction is an event in which a significant, widespread, and rapid decrease in the biodiversity on Earth occurs, often due to catastrophic environmental changes. Extinction Event: A significant, widespread, and rapid decrease in the biodiversity on Earth, often characterized by the disappearance of a large number of species in a relatively short period.