Scientists have searched for many years for a method to tell whether a volcano(火山)explosion will be small or large. Now，four scientists say they have discovered something in lava(熔岩)that will help do this. They did the research on Mount Unzen on the southern Japanese island of Kyushu.
Mount Unzen exploded in November，1990. Since then，more explosions have killed 43 people and destroyed the homes of more than 2，000 others. The volcano is still active today.
The scientists say the lava of the volcano contains high level of an element(成份)，which is a sign of the presence of the lava from deep in the earth，not from the surface. They say it shows that the lava coming directly from inside the earth is connected with huge explosions instead of smaller ones.
The scientists examined the lava from 18 explosions of Mount Unzen that took place during many thousands of years. They also studied the lava from other volcanoes in the area that are older than Mount Unzen. The lava studied came from both large and small explosions. The scientists found that large explosions contained more of the element than small ones. The scientists say if they had known this two years earlier，they could have warned the people living in the area near Mount Unzen and saved many lives.
5. From the passage we learn that Mount Unzen is the name of ______.
A. a huge volcano explosion
B. a Japanese island
C. an active volcano
D. a kind of lava
6. The four scientists discovered that the level of the element contained in the lava could tell ______.
A. the degree of a volcano explosion
B. the place of a possible explosion
C. the number of volcano explosions
D. the time of a possible explosion
7. The research of the four scientists was based on ______.
A. the lava that caused explosions
B. the presence of the lava from the surface
C. the damages caused by the 1990 explosion
D. the lava from different explosions in the area