阅读理解。 When people lose legs after accidents or illnesses, emergency care and artificial limbs(假肢)often allow them to walk again. Newts (蝾螈)in the same situation, on the other hand, can growlimbs back on their own! Scientists have known for a long time that certain animals can regrow limbs, but they haven't quite figured out how these creatures do it. Researchers have now come up with some new ideas. Their work may give people the ability to regrow lost limbs. The researchers started with two experiments: when you cut a newt's leg at the ankle, only the foot grows back; when you cut off a leg at the very end, the whole leg grows back. In both cases, the regrowth begins with stem cells. Stem cells can develop into nearlyany type of cell in the body. How do a newt's stem cells know when to grow only a foot and when to regrow a whole leg? This question relates to another mystery. In newt's, a cutoff leg will grow back only if the nerve bundle(神经束) in it also grows back, but if something prs the nerve bundle from growing thestem cells at the wound won't regrow a new leg. In its study, a British team focused on a protein called nAG. When the team pred nerves in a limb from growing, but added the nAG protein to stem cells in the limb, the limb still regrew. That protein seems to guide limb regrowth. People have proteins that are similar to nAG. Further researchinto these materials may someday help human limbs recover by themselves. 1. A newt will regrow its leg if________. A. its ankle was cut off but the nerve bundle was goodB. its leg was cut off and the nerve bundle stopped growingC. its leg was cut off and the nerve bundle could grow backD. its ankle and the nerve bundle could grow back 2. What do the underlined words "these materials" refer to? A. Nerve bundles and proteins. B. Proteins similar to nAG.C. Stem cells and proteins. D. Stem cells and nerve bundles. 3. The first sentence of the passage________. A. acts as a leadinB. shows where researchers got their new ideasC. states the author's opinionD. describes the result of researchers' studies 4. What's the purpose of studying the newts? A. To find out whether newts would regrow after being cutting off.B. To find out the similarity between human and newts.C. To find out what is nAG.D. To find a way of helping human limbs recover by themselves.