When death came, it was silent and sudden. On the night of 21 August, 1986, a cold and mis

ty blanket of carbon dioxide gas flowed through the valleys that run down from Lake Nyos in northwest Cameroon(喀麦隆). Hugging the ground, and smothered(窒息) more than 1,700 people, many as they slept.

The region surrounding Lake Nyos is volcanic, and so sulphurous(硫磺的) volcanic gases were initially suspected as the cause of the disaster. But subsequent investigation by teams of scientists dispatched to the remote highland lake suggested that as much as 80 million cubic metres of CO2 formerly held in solution in the lake's depths, has been released explosively at its surface, such an event, called a limnic(湖泊的) eruption, had been recorded only once before-at nearby Lake Manoun, 35 kilometers from Lake Nyos. That eruption, which killed 37 people in 1984, was kept secret at first, as the Cameroonian authorities had suspected terrorist involvement.

Expeditions to Lake Nyos since the 1986 disaster have allowed researchers to unravel(拆开) the geological process that underlie the lake's lethal tendencies. Armed with this information, a small group of scientists and engineers-funded to the tune of $450,000 by the Office of US Foreign Disaster Assistance, with additional help from the French and Cameroonian governments arrived in the area in January to attempt to 'degas' the lake, and so prevent it killing again. The plan is simple—the limited budget and the lack of decent roads into the region mean it has to be. The team will lower a length of high-density polyethylene pipe deep into the lake and pipe CO2 rich water towards the surface, where the gas can bubble out on a controlled way.

Most experts agree that degassing the lake is a good idea. But some say there is a small chance that the effort could trigger another limnic eruption and lives at risk. Although villagers surrounding Lake Nyos have been evacuated(撤退) to avoid a repeat of the 1986 catastrophe, farmers and their livestock still venture into the area, lured by its lush pastures.

Why was the death regarded silent？

A．Because it's quiet after an explosion.

B．Because the people were killed outright.

C．Because when it came, the victims were in sleep.

D．Because it happened suddenly.