【分析解答题】
1 Virtually all animals experience pain. Pain is a distress call from the body signaling some damaging stimulus or internal disorder. It is one of the most important sensations because it is translated into a negative reaction, such as withdrawal from danger. Rare individuals who are born without the ability to feel pain may die from such conditions as a ruptured appendix because they are unaware of the danger.
2 Pain receptors are unspecialized nerve fiber endings that respond to a variety of stimuli signaling real or possible damage to tissues. Some groups of pain receptors respond to specific classes of chemicals released from damaged or inflamed tissue. When pain fibers respond to peptides released by injured cells, this is called slow pain. Fast pain responses--for example, a pinprick or hot or cold stimuli--are a more direct response of the nerve endings to mechanical or thermal stimuli.
3 There is no pain center in the cerebral cortex. However, discrete areas have been located in the brain stem where pain messages from various parts of the body terminate. These areas contain two kinds of small peptides, endorphins and enkephalins, which have activity similar to morphine or opium. When these peptides are released, they bind with specific opiate receptors in the midbrain, decreasing the perception of pain.
Look at the four squares, 
, 
, 
, and 
, which indicate where the following sentence could be added to the passage. Where would the sentence best fit
They respond to stimuli such as changes in temperature and pressure, and movement of the tissue.
Pain receptors are unspecialized nerve fiber endings that respond to a variety of stimuli signaling real or possible damage to tissues. 
Some groups of pain receptors respond to specific classes of chemicals released from damaged or inflamed tissue. 
When pain fibers respond to peptides released by injured cells, this is called slow pain. 
Fast pain responses--for example, a pinprick or hot or cold stimuli--are a more direct response of the nerve endings to mechanical or thermal stimuli. 
网考网解析：
试题答案：A 答案解析：In the added sentence, They refers to Pain receptors, the subject of the previous sentence. In the added sentence, such as is a transition that introduces examples of a variety of stimuli, mentioned in the previous sentence.
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