共用题干
第三篇

The Exploding Lakes of Cameroon

What comes to mind when you think of a lake?You probably imagine a pretty scene
with blue water,birds,and fish.For the people in the northwestern Cameroon,however,
the image is very different.For them,lakes may mean terrible disasters.In 1984,
poisonous gases exploded out of Lake Monoun and came down into the nearby villages,
killing thirty-seven people.Two years later,Lake Nyos erupted.A cloud of gases rolled
down the hills and into the valleys and killed 1,700 people.
Lake Nyos and Lake Monoun are crater（火山口）lakes. They were formed when
water collected in the craters of old volcanoes.The volcanoes under Lake Nyos and Lake
Monoun are not active anymore.However,poisonous gases from the center of the earth
continue to flow up through cracks in the bottom of the lake.This is normal in a crater lake.
In most crater lakes,these gases are released often because the water"turns over"
regularly.That is,the water from the bottom of the lake rises and mixes with the water at
the top,allowing the gases to escape slowly.
However,in Lakes Nyos and Monoun,there is no regular turning over.No one knows
the reason for this fact,but as a result,these lakes have more gases trapped at the bottom
than other crater lakes.In fact,scientists who have studied Lakes Nyos and Monoun have
found 16,000 times more gases.When a strong wind,cool weather,a storm,or a
landslide（滑坡）causes the water to turn over suddenly,the gases escape in a violent
explosion.
In the past,no one knew when the gases might explode,so there was no way for the
villagers to escape disaster. Now scientists from the United States,France,and
Cameroon have found a way to reduce the gas pressure at the bottom of Lake Nyos.They
stood a 672-foot plastic pipe in the middle of the lake,with one end of the pipe near the
bottom and the other end in the air. Near the top of the pipe,the team put several holes
that could be opened or closed by a computer.Now,when the gas pressure gets too high,
the holes are opened and some of the gas-filled water shoots up through the pipe into the air
like a fountain.With less pressure,a disastrous explosion is much less likely.However,
the scientists are not sure that one pipe will be enough to prevent explosions.They hope to
put in others soon and they plan to install a similar pipe and a computer system at Lake
Monoun as well.
To protect people nearby until all of the pipes are in place,the scientists have installed
early warning systems at both lakes.If the gas pressure rises to a dangerous level,
computers will set off loud sirens（警报）and bright lights to warn the people in the villages.
That way,they will have time to escape from the dangerous gases.

Lake Nyos and Lake Monoun explode because
A:the gases rise to the top and mix with air.
B:people from the villages turn over the water.
C:scientists have put in a computer system.
D:they have more gases trapped at the bottom than other crater lakes.