1. (2022高一下·静安期中) For each blank in the following passage there are four words or phrases marked A, B, C and D. Fill in each blank with the word or phrase that best fits the context.

In 1965, 17-year-old high school student Randy Gardner stayed awake for 264 hours to see how he'd cope without sleep. On the second day, his eyes stopped focusing. Next, he lost the ability to 1 objects by touch. By day three, Gardner was moody and uncoordinated. At the end of the experiment, he was 2 to concentrate, had trouble with short-term memory and started hallucinating(出现幻觉). Although Gardner recovered without long-term psychological or physical 3, for others, sleeplessness can result in hormone(激素) imbalance, illness, and even death.

Sleep is 4. Adults need 7 to 8 hours of sleep at night, and adolescents need about 10. In the United States, it's estimated that 30% of adults and 66% of adolescents are 5 sleep deprived. When we lose sleep, learning, memory, mood, and reaction time are affected. Sleeplessness may also cause inflammation, hallucinations, high blood pressure, and it's even been 6 to diabetes and obesity.

How can sleep 7 cause such enormous suffering? Scientists think the answer lies with the 8 of waste products in the brain. During our waking hours, our cells are busy using up our day's energy sources which get broken down into various by-products, including adenosine(腺苷酸). As adenosine builds up, it increases the 9 to sleep, also known as sleep pressure. In fact, caffeine works by 10 adenosine's receptor pathways. Other waste products also build up in the brain, and if they're not cleared away, they collectively 11 the brain and are thought to lead to the many negative symptoms of sleep deprivation.

So, what's happening in our brain when we sleep, to prevent this? Scientists found something called the Lymphatic System, a clean-up mechanism(机能) that removes this build-up and is much more 12 when we're asleep. It works by using cerebrospinal fluid(脑脊髓液) to flush away toxic by-products that accumulate between cells. Lymphatic vessels, which 13 pathways for immune cells, have recently been discovered in the brain, and they may also play a role in 14 the brain's daily waste products. While scientists continue exploring the restorative mechanisms behind sleep, we can be sure that sleeping is a(n) 15 if we want to maintain our health and our sanity.