Read the passage and choose the correct answer.

Every year, the average temperature of the Earth's surface gets a little bit warmer. This gradual trend is called global warming. Warmer weather may sound nice, but global warming is something to be very concerned about.

Scientists are worried that continued warmer temperatures could damage the environment in many devastating ways. Rising temperatures might cause plants and animals to become extinct. They could melt enough polar ice to cause the levels of the sea to rise. Weather patterns could also change. There might be more droughts or serious storms with flooding. In some areas, human diseases could spread.

What is causing global warming? Humans are mostly to blame. Pollution from factories and cars add toxic gases to the air. These gases rise to the Earth's atmosphere. As the sun’s rays warm the Earth, the gases work much like the glass in a greenhouse. They help trap the heat in the atmosphere and make the Earth grow warmer. That is why they are nicknamed “greenhouse gases.”

It is our responsibility to take care of our planet. Global warming is a serious problem with serious consequences. If we want future generations to enjoy their time on Earth, we must act now!

From the description of greenhouse gases, it can be concluded that a greenhouse is __________.