What is the main topic of the passage?

Câu hỏi: Read the following passage and mark the letter A, B, c, or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42.
We are descendents of the ice age. Periods of glaciations have spanned the whole of human existence for the past 2 million years. The rapid melting of the continental glaciers at the end of the last ice age spurred one of the most dramatic climate changes in the history of the planet. During this interglacial time, people were caught up in a cataclysm of human accomplishment, including the development of agriculture and animal husbandry. Over the past few thousand years, the Earth's climate has been extraordinarily beneficial, and humans have prospered exceedingly well under a benign atmosphere.
Ice ages have dramatically affected life on Earth almost from the very beginning. It is even possible that life itself significantly changed the climate. All living organisms pull carbon dioxide out of the atmosphere and eventually store it in sedimentary rocks within the Earth's crust. If too much carbon dioxide is lost, too much heat escapes out into the atmosphere. This can cause the Earth to cool enough for glacial ice to spread across the land.
In general the reduction of the level of carbon dioxide in the atmosphere has been equalized by the input of carbon dioxide from such events as volcanic eruptions. Man, however, is upsetting the equation by burning fossil fuels and destroying tropical rain forests, both of which release stored carbon dioxide. This energizes the greenhouse effect and causes the Earth to warm. If the warming is significant enough, the polar ice caps eventually melt.
The polar ice caps drive the atmospheric and oceanic circulation systems. Should the ice caps melt, warm tropical waters could circle the globe and make this a very warm, inhospitable planet.
Over the past century, the global sea level has apparently risen upwards of 6 inches, mainly because of the melting of glacial ice. If present warming trends continue, the seas could rise as much as 6 feet by the next century. This could flood coastal cities and fertile river deltas, where half the human population lives. Delicate wetlands, where many marine species breed, also would be reclaimed by the sea. In addition, more frequent and severe storms would batter coastal areas, adding to the disaster of the higher seas.
The continued melting of the great ice sheets in polar regions could cause massive amounts of ice to crash into the ocean. This would further raise the sea level and release more ice, which could more than double the area of sea ice and increase correspondingly the amount of sunlight reflected back into space. The cycle would then be complete as this could cause global temperatures to drop enough to initiate another ice age.
What is the main topic of the passage?
A. The possibility that the popular ice caps will melt.
B. Man's effect on the carbon dioxide level in the atmosphere.
C. The climate of the Earth over the years.
D. The coming of another ice age.