Many scientists say that the world is getting hotter and hotter. If they are right, the ice at the North and South Poles will melt(融化) eventually. If this                 (happen), the sea will rise and may cover most or all of Singapore. This                 (force) everybody to move to a safer country.

        We know that the sun                 (heat) the earth. The heat from the sun hits the ground, and some of it goes back into the sky. However, factories and vehicles(车辆) produce a lot of harmful gases. When the gases rises into the sky, heat from the sun can come through them but heat from the ground cannot. It stays near us in the sky and makes the air hotter and hotter every year.

        Scientists call this the “greenhouse effect” because a greenhouse is a small glass building which is hot inside. It                 (use) for growing plants in cool countries. Many countries are worried about the greenhouse effect. They                 (try) to stop factories and vehicles from                 (send) harmful gases into the sky. Scientists are not sure when the world will get much hotter. Some people say that the greenhouse effect                 (start) already and has begun to affect the world's weather.