Everyone talks about the greenhouse effect  and how our Earth and its climate  are being affected. We can do a science experiment designed by some kids to show this effect and what is happening to the Earth. First prepare the following items: *2 glass containers  that need to be the same size and shape  *1 gallon plastic bag  *4 cups of very cold water  *Ice cubes Fill each container with two cups of cold water. Add the same amount of ice cubes to each container. Then put one of the containers in the plastic bag and close it tight. Put both containers in the sun together and keep them there for at least an hour. At the end of one hour remove  the container from the plastic bag and immediately check the temperature of the water. After you record this, find out the temperature of the water that is not covered. When you place the containers in direct sunlight, the air inside the bag becomes very warm. The heat filters   in but cannot escape so it stays in the closed plastic bag and increases the temperature. What if you leave that for two, three or four hours? Think about what is happening to the Earth. There are chemicals that are creating a type of plastic bag around the Earth. It causes our air to get warmer because of the heat from the sun being kept in this closed area. Think about your car in the hot summer. This is what happens when you leave your car outside all day. This greenhouse effect has caused your car to be so hot that you sometimes cannot even touch the steering wheel. This science project goes to show what affects our planet. According to the text, the greenhouse effect appears mainly because of   _  .
the hot sunlight chemicals around the Earth the plastic bag human activities