Read the following passage and mark the letter A, B, C, or D to indicate the answer to each of the question. Anthropogenic global warming is a theory explaining today's long-term increase in the average temperature of Earth's atmosphere as an effect of human industry and agriculture. Since the latter half of the 20th century, growing banks of data and improved climate models have convinced most climate scientists that rising trends in greenhouse gas emissions are directly responsible ...

Read the following passage and mark the letter A, B, C, or D to indicate the answer to each of the question.

Anthropogenic global warming is a theory explaining today's long-term increase in the average temperature of Earth's atmosphere as an effect of human industry and agriculture.

Since the latter half of the 20th century, growing banks of data and improved climate models have convinced most climate scientists that rising trends in greenhouse gas emissions are directly responsible for a rising trend in atmospheric temperature. The source of these emissions varies, consisting of a mix of gases that include methane and carbon dioxide. While some sources - such as volcanoes - are natural, their overall emissions compared with those produced by human industries, transport, and livestock have been regarded as insignificant over recent centuries.

Greenhouse gases are made of molecules that absorb electromagnetic radiation, such as the light reflecting from the planet's surface, and re-emit it as heat. These gases include methane, carbon dioxide, water, and nitrous oxide. Despite making up only a small percentage of the atmosphere's mix of gases they are very important. If we had no naturally occurring greenhouse gases in the atmosphere at all, the average global temperature on Earth would be a much lower -18 degrees instead of the roughly 15 degrees Celsius we have enjoyed most of human history.

Carbon dioxide levels have steadily risen over the past two centuries, thanks largely to the burning of fossil fuels for electricity general, transportation, and smelting. Current levels are approximately 415 parts per million (ppm), up from pre-industrial levels of around 280 ppm. Models vary in their predictions of further temperature increases, and depend heavily on future trends in greenhouse gas emissions. Conservative estimates by the Intergovernmental Panel on Climate Change predict an even chance of 4 degrees Celsius rise by the end of the century if current emissions trends continue.

According to paragraph 3, what would happen if the Earth were bereft of natural greenhouse gases?