請依下文回答第 46 至 50 題： The word volcano can describe anything from a single opening in the earth a few meters across to a field of connected openings that might be 20 kilometers or more in width. Most researchers consider any set of openings that are connected to a large central core as a single “volcano”. Otherwise, they would have to deal with an endless number of names for each individual opening. Italy’s Stromboli has been almost constantly erupting for over 2400 years, but most eruptions do not last that long. Currently, there are about 15 volcanoes worldwide that have been actively erupting for the last 30 years. On the other end of the scale, about ten percent of volcanoes have eruptions which last for just a single day. The majority of the eruptions lasted three months or less. This means the average time from beginning to end is usually about seven weeks. From the number of volcanic eruptions detected over the past few hundred years, it would appear that they are becoming much more frequent over time. However, when researchers tracked the growth in reported eruptions along with the growth of the human population, they realized that volcanic activity wasn’t actually becoming more frequent. Instead, as people spread over more areas of the earth’s surface, the monitoring and reporting of volcanic activity inevitably increased with the expanding population. The highest volcano when measured from sea level to its top is Ojos del Salado in the Andes mountains in South America. It is 6,887 meters above sea level. However, when measured from its base on the surface of the earth to the top, a different set of volcanoes become the highest ones. Some volcanoes in the Mauna Loa zone near Hawaii rise as much as 19 kilometers above their bases at the bottom of the Pacific Ocean. The first thing to do when caught near a volcanic eruption is to avoid a potentially disastrous situation, listen to the radio and travel only as instructed by the local police or other authorities. Stay away from lava, mud and flying rocks. Avoid river areas and low-lying regions. Before you leave the house, put on a long-sleeved shirt, long pants, and eyeglasses or sunglasses to protect your eyes. Wear an emergency mask or hold a wet cloth over your face if you do not have one. Since ash can damage engine parts, refrain from driving if possible. If you decide not to leave the house, close all windows and doors to prevent ash from getting inside.