A shell is considered as biotic (living) because it is a hard, rigid covering of many animals such as snail, sepia, pila, turtles, sea urchins. A shell is protective outer layer of these soft bodied animals.
Answer:
The answer is oxygen and glucose
The correct option is C.
Living organisms that live in the littoral zones in the ocean are used to frequent changes in temperature and in the salinity of the ocean water. For an organism to survive in this environment, it needs to have adaptive features that will increase its survival. The littoral zone is divided into three zones, which are high, middle and lower littoral zones. Organisms living in high littoral zone have adaptive features that make them more adapted to desiccation due to the long hours of sunlight to which they are exposed. The organisms are usually exposed directly to the air or they can be enclosed in burrows.
The Precambrian era envelopes the major bulk of the history of the Earth, beginning from the creation of the planet approx 4.5 billion years ago and terminating with the origination of composite, multicelled forms of life approximately 4 billion years after.
The Precambrian refers to the earliest of the geologic ages that are signified by the distinct layers of sedimentary rock. The Earth was about more than six hundred million years old when life started. The planet had cooled down from its native molten state, creating a solid crust and oceans formed by water vapor in the atmosphere.
At about three billion years ago, the atmosphere of the Earth was virtually devoid of oxygen. At approximately 2.4 billion years ago, oxygen was discharged from the seas as a waste product of photosynthesis by cyanobacteria. The levels of the gas slowly raised, attaining about 1 percent around two billion years ago.
Approximately 800 million years ago, the levels of oxygen attained about 21 percent and started to breathe life into more composite species. The oxygen-rich ozone layer was also created, protecting the surface of the Earth from the harmful solar radiation.
