Question 21 is based on the recording you have just heard.

听力原文

    In 1984, the World Chess Championship was called off abruptly due to the withered frame of a player who was competing for the title. He wasn’t alone in experiencing extreme physical effects of the game. Elite players can reportedly burn up to an absurd six thousand calories in one day.

    Does that mean that thinking harder is a simple route to losing weight? Well, (19) [when the body is at rest, we know that the brain uses up a startling 20% to 25% of the body’s overall energy.] This level of utilization actually makes the brain the most energy-expensive organ in the body, and yet it makes up only 2% of the body’s weight overall.

    So the more we put this organ to work, the more calories we’ll burn? Technically, the answer is yes, for cognitively difficult tasks. What counts as a “difficult” mental task varies between individuals. But generally, it could be described as something that “the brain cannot solve easily using previously learned routines, or tasks that change the conditions continuously”. However, deep thinking will not burn off the calories gained from eating a sugary snack. Because in relation to the brain’s huge overall energy usage, which is devoted to a multitude of tasks, the energy required just to think harder is actually comparatively tiny. (20) [We are unconscious of most of what uses up the brain’s energy—a lot of that activity is unrelated to conscious activities like learning how to sing or play the guitar.]

    (21) [The brain is able to allocate blood and thus energy to particular regions that are being active at that point. But the overall energy availability in the brain is thought to be constant.] So, while there might be significant increases in energy use at localized regions of the brain when we perform difficult cognitive tasks, when it comes to the whole brain’s energy budget overall, these activities don’t significantly alter it.

    So why did the chess champion grow too skinny to compete in his chess competition? The general consensus is that it mostly comes down to stress and reduced food consumption, not mental exhaustion. Keeping your body pumped up for action for long periods of time is very energy-demanding. If you can’t eat as often or as much as you can or would normally, then you might lose weight.

21. What do we learn about the overall energy availability in the brain?（关于大脑总体的能量供应，我们可以了解到什么？）

解析：A。录音结尾提到，当我们执行高难度的认知任务时，大脑局部区域的能量消耗可能会显著增加，但整个大脑的能量消耗并不会因这些活动而产生显著变化。也就是说，大脑使用的总体能量是基本恒定的，A项内容与此相符，其中is believed to remain是对录音中is thought to be的同义替换，constant为原词复现，故A项为正确答案。

错项排除：全篇录音围绕思考活动和能量消耗展开，但没有提到大脑总体的能量使用是一切心理活动的必要条件（prerequisite），B项只是利用录音中出现的mental和activity进行拼凑，故排除。录音结尾虽然再次提到国际象棋选手消瘦的原因，但只是分析了可能的原因：压力太大、进食减少，并不能说明大脑的能量消耗能帮助缓解精神疲劳，也不能得出能量供应与进食相关，故C、D两项排除。