填空题

阅读下面这篇短文，短文后有2项测试任务；(1)第23～26题要求．从所给的6个选项中为第2～5段每段选择1个正确的小标题；(2)第27～30题要求从所给的6个选项中选择4个正确选项，分别完成每个句子。
Icy Microbes
1 In ice that has sealed a salty Antarctic lake for more than 2,800 years, 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.
2 A research team led by Peter Doran of the University Of Illinois at Chicago drilled through more than 39 feet 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.
3 Doran said the microbes have been age-dated at 2,800 years 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.
4 Called Lake Vida, the 4.5-square-kilometer body is one of a series of lakes located in the McMurdo Dry Valleys of Antarctica, some 2,200 kilometers 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 62 feet below there was a pool of liquid water that was about seven times more salty than seawater.
5 That prompted the researchers to return in 1996 with 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.
6 The researchers will return in 2004 equipped with instruments that are sterilized. They will then drill through the full 62 feet 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. Paragraph 3 ______