阅读理解：
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.
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 tapped 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 landslidecauses the water to turn over suddenly，the gases escape in a violent explosion.
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 sirensand bright lights to warn the people in the villages.That way, they will have time to escape from the dangerous gases.
A Water will flow down the hills
C A strong wind will rise from the lakes
42 Which of the following statements about Lake Nyos and Lake Monoun is true?
B They are at the top of two active volcanoes.
D Water in them turns over regularly.
A the gases rise to the top and mix with air.
C scientists have put in a computer system.