—Oh，my income's rather variable，but I earn 100 yuan a day________.

            As scientists learn more about the causes of earthquakes, they become better able to predict them. First of all, they know that quakes are1to occur in areas where the tectonic(地壳构造的)plates push2each other. They3these areas closely, using different instruments. They4the vibrations(震动)of the earth with seismographs(地震仪), which5on rolls of paper all movements, both large and small. Scientists usually use the Richter scale(里氏震级) ， 6after a famous seismologist, for these measurements. When an earthquake measures over 4.5 on the Richter scale, it is7enough to cause damage. Scientists have determined that a(an)8is not likely to have a large9when many small quakes have been10. When the small quakes stop, then pressure11and a strong quake is more likely.

          There are several other12to predict earthquakes. Scientists can measure the tiny13in the tilt(倾斜)of the land that happen before an earthquake occurs. Increases in the amount of radon(氡), a substance found in deep14 ， as well as a high level of water in the wells, can also15that an earthquake is likely to occur. In addition, scientists have found that some animals, like dogs, behave16before an earthquake: they become17and howl.

           Seismologists have18_predicted several earthquakes, such as the earthquake which occurred in Zhangjiakou, China in 1998. 19 ，  not all earthquakes are predictable, and a system for predicting is not yet20.

—There's no________for this while you are on duty.