Earthquakes and volcanic eruptions usually cause the largest, most disastrous debris ( ) flows. A great many victims of the earthquake that struck Wenchuan of Sichuan province last May were killed not by the quake itself but by the sudden large amounts of debris that followed. Most debris flows occur where the soil is composed mainly of sand or colluviums ( ) . Loose soils made up of bits of sand or colluviums fall apart easily and are main material for fast -moving debris flows. Scientists have conducted endless experiments to find out how debris flows work. They have learned that the amount and the timing of water entering the soil are crucial ( ). Typically, that means debris flows often occur when a long rainy period of many weeks is followed by an intense, brief heavy rain.                              . More often than not, however, debris flows are started by water, in the form of rain, snow, or river flow. In 1998, heavy rains from Hurricane Mitch unleashed about one million tons of mudslides, burying thousands of people, in Central America. Water flows through the soil until it hits a hard layer of bedrock or clay. There the water builds up, filling the tiny spaces between the soil and rock. That buildup reduces the natural friction ( ) that keeps the rock and soil debris lying on the slope. If conditions are approaching dangerous levels, the slope will then fall down. Despite what scientist know about the fact, debris flows are still impossible to predict at present because there are many other uncertains affect the stability of each slope. However, one thing is known: On hillsides where one debris flow has occurred, another will eventually follow. The next debris flow could take days, weeks, or years to happen. Debris flows usually take place   _  .
during earthquakes and volcanic eruptions after a heavy rain or an intense snowstorm when there is a right amount of water entering the soil when a long rainy period is followed by a heavy rain
Question: Earthquakes and volcanic eruptions usually cause the largest, most disastrous debris ( ) flows. A great many victims of the earthquake that struck Wenchuan of Sichuan province last May were killed not by the quake itself but by the sudden large amounts of debris that followed. Most debris flows occur where the soil is composed mainly of sand or colluviums ( ) . Loose soils made up of bits of sand or colluviums fall apart easily and are main material for fast -moving debris flows. Scientists have conducted endless experiments to find out how debris flows work. They have learned that the amount and the timing of water entering the soil are crucial ( ). Typically, that means debris flows often occur when a long rainy period of many weeks is followed by an intense, brief heavy rain.                              . More often than not, however, debris flows are started by water, in the form of rain, snow, or river flow. In 1998, heavy rains from Hurricane Mitch unleashed about one million tons of mudslides, burying thousands of people, in Central America. Water flows through the soil until it hits a hard layer of bedrock or clay. There the water builds up, filling the tiny spaces between the soil and rock. That buildup reduces the natural friction ( ) that keeps the rock and soil debris lying on the slope. If conditions are approaching dangerous levels, the slope will then fall down. Despite what scientist know about the fact, debris flows are still impossible to predict at present because there are many other uncertains affect the stability of each slope. However, one thing is known: On hillsides where one debris flow has occurred, another will eventually follow. The next debris flow could take days, weeks, or years to happen. Debris flows usually take place   _  .
A. during earthquakes and volcanic eruptions
B. after a heavy rain or an intense snowstorm
C. when there is a right amount of water entering the soil
D. when a long rainy period is followed by a heavy rain
Answer: