Read the following passage and choose the best answers to the given questions

 Read the following passage and choose the best answers to the given questions.
The "greenhouse effect" is the warming that happens when certain gases in Earth’s atmosphere trap 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, where it is absorbed and then radiates back into the atmosphere as heat. In the atmosphere, “greenhouse” gases trap some of this heat, and the rest 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 livable. Without it, the Earth's surface would be an average of about 60 degrees Fahrenheit cooler.
In 1895, the Swedish chemist Svante Arrhenius discovered that humans could enhance the greenhouse effect by making carbon dioxide, a greenhouse gas. He kicked off 100 years of climate research that has given us a sophisticated understanding of global warming.
Levels of greenhouse gases (GHGs) have gone up and down over the Earth's history, but they have been fairly constant for the past few thousand years. Global average temperatures have stayed fairly constant over that time as well, until recently. Through the burning of fossil fuels and other GHG emissions, humans are enhancing the greenhouse effect and warming the Earth.
Scientists often use the term "climate change" instead of global warming. This is because as the Earth's average temperature increases, 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. As a result, the climate changes differently in different areas.

 44. The word "livable" in the passage is used to describe that ………… 

A. the Earth's surface is above 60 degrees Fahrenheit

B. the Earth's climate is full of carbon dioxide

C. the Earth's climate is too cold

D. the Earth's climate is suitable for living things

 45. Which of the following is NOT true according to the passage?

A. The greenhouse effect takes place only in some places on Earth, where the temperature is too low.

B. The greenhouse effect has recently become stronger due to humans' activity of burning fossil fuels.

C. The amount of heat trapped increases with the number of the greenhouse gases in the atmosphere.

D. Without greenhouse effect, humans and other living things would not be able to live on Earth.

 46. The word “they” refers to………

A. levels of greenhouse gases (GHGs)

B. the Earth's history

C. the burning of fossil fuels

D. global average temperatures

 47. The phrase "the rest" in the passage refers to …………

A. sunlight   B. heat

C. gases     D. light

 48. In what way are humans enhancing the greenhouse effect and warming the Earth?

A. Burning of fossil fuels and other GHG emissions

B. Kicking off 100 years of climate research

C. Changing the amount of rain and snow falling

D. Cooling some areas and warming others

 49. The passage mainly discusses …………

A. the greenhouse effect B. greenhouse gases

C. global warming D. climate changes

 50. Which of the following is TRUE about greenhouse effect according to the passage?

A. It was known well before 1824.

B. It is commonly caused by oxygen in the atmosphere.

C. It makes the Earth's climate too cold for living things.

D. It has been a process that causes global warming.