填空题

Perched among the highlands of western Cameroon, bordered by green mountains and cliff laces, Lake Nyos is a scene of breathtaking beauty. But the picture cannot be (36) . A detailed study reveals that the lake could release a (37) cloud of carbon dioxide, capable of wiping out entire communities around its shores.
Tile warning, from a team of scientists, comes nearly 20 years after the lake (38) an estimated 80m cubic metres of CO₂ into the atmosphere. Heavier than air, the cloud of gas rolled down, (39) hillsides and villages. Silent and invisible, it (40) the air of oxygen, killing hundreds of cattle and claiming the lives of more than 1,700 people up to 26km away. "It was one of the most mysterious (41) scientists have ever investigated," said George Kling, an ecologist at the University of Michigan.
Researchers called in after the 1986 (42) discovered that the lake, which sits atop a volcano, contained record levels of carbon dioxide. Gas bubbling up from the Earth’s magma was under such (43) at the bottom of the 200-metre-deep lake that it dissolved until it reached max. (44) .
To prevent a recurrence, in 2001 engineers installed a pipe to suck CO₂ from the bottom of the lake and release it harmlessly into the air. (45) . Dr Kling’s team recommends the urgent installation of a further tbur pipes in each lake at a rate of one a year. (46) .