Answer:
D. They define geological periods because scientists use them as reference points for tracking the history of the earth.
Explanation:
Catastrophic events related to changes in geological timeline are very useful in defining geological periods.
These events have very significant appearance in the geological record and can be used to determine the history of the earth.
The catastrophic events usually culminates into extinction cycles or termination of some organisms from the earth surface.
Geologists use these events that are often wide spread to delineate geological periods.
Cell wall is the part that protects the cell from things like viruses (but usuallly fails)
It is made of cell tissue made by mitochondria.
The benefits of the sun for the plant Life is not upright on Earth in the absence of plants, as it is the first and main source of food for both human and animal, therefore, it is worth mentioning that these plants need a set of conditions to grow and perhaps the most important of which is the availability of sun rays.