题目内容：
根据下面资料，回答题 Earthquake survivors trapped in rubble could one day be saved by an unlikely rescuer: A robotic caterpillar thatburrows its way through debris. Just a few centimeters wide, the robot relies on magnetic fields to propel it throughthe kind of tiny crevices that would foil the wheeled or tracked search robots currently used to locate people trappedin collapsed buildings.
The caterpillar's inventor, Norihiko Saga of Akita Prefectural University in Japan, will demonstrate his newmethod of locomotion at a conference on magnetic materials in Seattle. In addition to lights and cameras, a searchcaterpillar could be equipped with an array of sensors to measure other factors--such as radioactivity or oxygen lev-els-that could tell human rescuers if an area is safe to enter.
The magnetic caterpillar is amazingly simple. It moves by a process similar to peristalsis, the rhythmic contrac-tion that moves food down your intestine. Saga made the caterpillar from a series of rubber capsules filled with amagnetic fluid consisting of iron particles, water, and a detergent-like surfactant, which reduces the surface tensionof the fluid. Each capsule is linked to the next by a pair of rubber rods. The caterpillar' s guts are wrapped in aclear, flexible polymer tube that protects it from the environment.
To make the caterpillar move forwards, Saga moves a magnetic field backwards along the caterpillar. Inside thecaterpillar's "head" capsule, magnetic fluid surges towards the attractive magnetic field, causing the capsule tobulge out to the sides and draw its front and rear portions up. As the magnetic field passes to the next capsule, thefirst breaks free and springs forward and the next capsule bunches up. In this way, the caterpillar can reach speeds of4 centimeters per second as it crawls along.
Moving the magnetic field faster can make it traverse the caterpillar before all the capsules have sprung back totheir original shapes. The segments then all spring back, almost but not quite simultaneously.
Saga plans to automate the movement of the caterpillar by placing electromagnets at regular intervals along theinside of its polymer tube. By phasing the current flow to the electromagnets, he' ll be able to control it wirelessly
via remote control. He also needs to find a new type of rubber for the magnetic capsules, because the one he' s usingat the minute eventually begins to leak.
But crawling is not the most efficient form of locomotion for robots, says Robert Full of the University of Cali-fornia at Berkeley, an expert in animal motion who occasionally advises robotics designers. "If you look at the ener-getic cost of crawling, compared to walking, swimming or flying, crawling is very expensive, " he says. Walking,on the other every step, energy is conserved in the foot and then released to help the foot slating up.
Saga acknowledges this inefficiency but says his caterpillar is far more stable than one that walks, rolls onwheels or flies. It has no moving parts save for a few fluid-filled rubber capsules. Biped robots and wheeled robotsrequire a smooth surface and are difficult to miniaturize, and flying robots have too many moving parts. "My peri-staltic crawling robot is simple and it works, " he says.
From this passage, we can learn that _______. A．a robotic caterpillar can crawl by a pair of rubber rods
B．when a caterpillar moves, the magnetic field moves backwards aloug it
C．the environment couldn' t influence a robotic caterpillar' s guts, which are wrapped in a capsule
D．crawling is very stable and efficient, and when it moves, only a few elements are needed
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