A、No role. B、A small role. C、A declining role. D、A major role. B题目问的是手术对治疗背痛的作用。由定位句可知，手术对于治疗背痛的作用微乎其微，故选B。

(21) A lot of people think that sitting is easier on their back than standing or lifting. Not true. People whose jobs require them to sit for long periods of time suffer as much from back pain as people who lift all day long. (22) Many world-class researchers believe that the huge increase in back pain over the past couple of decades—and it is huge—has a lot to do with the fact that more and more of us are spending our work days in chairs. Many people have the notion that, if their back pain gets bad enough, they can always resort to surgery. Nothing could be further from the truth. (23) The amount of pain someone is in has very little to do with whether or not he or she could benefit from surgery. One British researcher has estimated that for every 10,000 people who experience back pain, only four need surgery. Not very many years ago, back pain patients were routinely put to bed, sometimes for weeks or months. Two or three days of bed-rest is now the norm. After that, people are advised to return to their normal activities, gradually if necessary. The reasons for the 180°shift are what’s interesting. (24) For starters, if you stay in bed, your muscle strength can decline by as much as three percent a day. (25) In our next lecture, we will look at the relationship between depression and chronic back pain, where a lot of attention and research have been done to relate the two.
21．What myth does the lecturer dispel at the beginning?
22．What might be the cause of the increase in back pain in recent years?
23．What role does surgery play in combating back pain?
24．Why don’t back pain patients put to bed for more than two or three days?
25．What will be the next topic?