单项选择题
【案例分析题】
Icy Microbes
1In ice that has sealed a salty Antarctic lake for more than 2，800years，scientists have found frozen bacteria and algae that returned to life after thawing.The research may help in the search for life on Mars，which is thought to have subsurface lakes of ice.
2A research team led by Peter Doran of the University Of Illinois at Chicago drilled through more than 39feet of ice to collect samples of bacteria and algae.When Doran’s team brought them back and warmed them up a bit，they sprang back to life.
3Doran said the microbes have been age-dated at 2，800years old，but even older microbes may live deeper in the ice sheet sealing the lake，and in the briny water below the ice.That deeper ice and the water itself will be cautiously sampled in a later expedition that will test techniques that may one day be used on Mars.
4Called Lake Vida，the 4.5-square-kilometer body is one of a series of lakes located in the McMurdo Dry Valleys of Antarctica，some 2，200kilometers due south of New Zealand.This lake has been known since the 1950s，but people ignored it because they thought it was just a big block of ice.While at the site for other research in the 1990s，Doran and his colleagues sent radar signals into the clear ice covering the lake and were surprised to find that 62feet below there was a pool of liquid water that was about seven times more salty than seawater.
5That prompted the researchers to return in 1996with equipment to drill a hole down to within a few feet of the water layer.At the bottom of this hole，researchers harvested specimens of .algae and bacteria.
6The researchers will return in 2004equipped with instruments that are sterilized.They will then drill through the full 62feet of ice and sample some of the briny water from the lake for analysis.The water specimen will be cultured to see if it contains life.Specimens from the water are expected to be even older than the life forms extracted from the ice covering.
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