American scientists admitted that several problems remain in their prediction methods．For example, they said their methods have not shown how strong an eruption will be, or how long it will last．But the scientists said that continuing eruptions of Mount St．Helens are .helping them improve their methods．And they said the methods can be used to predict when other volcanoes will explode．
第 31 题 Since the 1980's, how many eruptions have happened in Mount St．Helens?

Predicting Volcanic Explosions
American scientists are studying Mount St．Helens volcano in the northwestern United States to learn when it might explode again．Their studies have improved methods for predicting volcanic explosions long before they happen.
Mount St．Helens exploded twice in May, 1980．These two powerful explosions
caused deaths and injuries and much property damage．Scientists did not know that the volcano would explode, so no warnings were given．But after the eruptions, scientists began studying changes in the earth's activity to improve prediction methods．In the following months, there were 13 smaller explosions at Mount St．Helens．Each time, scientists knew the volcano would erupt, so they were ahle to warn people living near Mount St．Helens to leave.
Scientists sharpened their prediction (预言) skills by studying the link between volcanoes and earthquakes．To do this, they placed a numher of devices within 20 kilometers of Mount St．Helens to measure deep earthquakes in the area．They also measured shallow quakes less than 3 kilometers below the crater (火山口 ), the huge hole at the top of the volcano．The scientists found that these shallow quakes increase in the days and weeks before an explosion．During this same period, melted rock and gas from deep inside the earth
build up under the volcano's dome, the small hill in the center of the crater..These causes the sides of the dome to push out．As the dome expands, cracks form. on the crater floor around it．Scientists found that the dome begins to expand 3 to 4 weeks before the volcano explodes, and the cracks on the crater floor widen．Because of their studies, scientists were able to predict 13 smaller explosions at Mount St．Helens several weeks before they happened．Sometimes they even knew the hours of the day when the volcano would explode.
American scientists admitted that several problems remain in their prediction methods．For example, they said their methods have not shown how strong an eruption will be, or how long it will last．But the scientists said that continuing eruptions of Mount St．Helens are .helping them improve their methods．And they said the methods can be used to predict when other volcanoes will explode.
第 31 题 Since the 1980's, how many eruptions have happened in Mount St．Helens?