Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks.

The "greenhouse effect" is the warming that happens when certain gases in Earth's atmosphere (24) ________ heat. These gases let in light but keep heat from escaping, like the glass walls of a greenhouse. First, sunlight shines onto the Earth's surface, (25) ______ it is absorbed and then radiates back into the atmosphere as heat. In the atmosphere, “greenhouse gases trap some of this heat, and the (26) ________ escapes into space. The more greenhouse gases are in the atmosphere, the more heat gets trapped.

Scientists have known about the greenhouse effect since 1824, when Joseph Fourier calculated that the Earth would be much colder if it had no atmosphere. This greenhouse effect is what keeps the Earth's climate (27) _________. Without it, the Earth's surface would be an average of about 60 degrees Fahrenheit cooler. Scientists often use the term "climate change" instead of global warming. This is because as the Earth's average temperature climbs, winds and ocean currents move heat around the globe in ways that can cool some areas, warm others, and change the amount of rain and snow falling. (28)_________, the climate changes differently in different areas.