A、It is believed to remain basically constant. B、It is a prerequisite for any mental activity. C、It is conducive to relieving me

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 the extreme physical effects of the game. Elite players can reportedly burn up to an absurd 6,000 calories in one day. Does that mean that thinking harder is a simple route to losing weight?
Well, when the body is at rest, we know that the brain uses up a startling 20%-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.
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. 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.
Questions 19 to 21 are based on the recording you have just heard.
19. What do we learn about the brain when the body is at rest?
20. What does the speaker say about the consumption of the brain’s energy?
21. What do we learn about the overall energy availability in the brain?