Answer: is option C: <em>Prolonged periods of cooling and warming</em>.
Explanation:
In the history of Earth, climate varies time to time. At times, the Earth's atmosphere was much hotter and humid as compare to the present time, but similarly it has been noticed that climate also has been much colder than he present time, whereas the number of glaciers covers much of the Earth's surface. There are two kinds of periods in which we further classified Earth's climate namely, Glacial period, and Inter-glacial periods. It has been noticed that the average global temperature of Earth during glacial periods was around 5.5°C or 10°F, which is less than Earth's present climate. On the other hand, during inter-glacial periods Earth's temperature was about 1.1°C or 2.0°F, which is again higher as compared to current temperature. Over the past 900,000 years, Earth's temperature varied less than 5°C. Scientist believe by looking at the Earth's climate history, that glaciers will proceed again in formation, but it will take thousands of years.
