Section B
Directions: There are 2 passages in this section．Each passage is followed by some questions or unfinished statements．For each of them there are four choices marked A, B, C and D．You should decide on the best choice.
The oceans, which cover nearly three-quarters of Earth's suce, remain largely unexplored because of their vastness and inaccessibility to us air breathers．Webb Research of East Falmouth, Massachusetts, hopes to open up much of that mystery field with a new underwater vehicle that is pushed ahead solely by the ocean itself, so it can potentially study the watery depths for years at a time．Such long-lasting detectors may someday form．fleets that provide up-to-minute data for weather forecasting.
In the past, such autonomous underwater vehicles, or AUVs, have been battery powered and therefore required frequent recharges that limited their distance and depth of travel．Webb's Slocum glider(滑翔机) hopes to solve this problem．'We conceived of a thermally powered engine to get the energy from the colder deep water of the ocean and the shallower warm water,' Clayton Jones, an engineer at Webb Research, explains．'We picked out a material that undergoes a state change between those two temperatures in the ocean.' When cold, this form-shifting substance is a compact solid．When warm, it expands and becomes a less dense liquid．The material's expansion in warmer waters charges an internal mechanism that acts like stored power to control its buoyancy: by pumping mineral oil in and out of an external air bag, it alters its volume but maintains its weight．'The glider takes up more volume and becomes lighter than the water around it--less dense,' Jones says.
Scientists can also fit the Slocum glider with a variety of different sensory equipment and Global Positioning Systems．Each time it suces, it can raise its tail antenna(天线) to transmit data to and receive instructions from researchers sitting comfortably in their cozy offices．The glider will have its first test in salt water at the end of this September.
The gliders could also get through and follow previously unstudied phenomena such as small underwater eddies (漩涡), which are vital to understanding water transport and heat flow in the ocean and therefore play an important role in storm formation and other weather development．'We have all these data snapshots(快照) from floats that just flow with the deep ocean current,' Jones says．'To be able to fly through eddy sections, then transport with the whole eddy itself as it moves through the ocean, would be a very nice phenomenon to take a look at.'
The new underwater vehicle is designed to ______.