Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

Students who stay up all night to cram for an exam are doing themselves more harm than good, according to research into the link between sleep and memory published yesterday.

Scientists at Harvard Medical School discovered that people who deprive themselves of sleep so that they can study until the last minute are unlikely to remember anything that would improve their performance, while suffering the crippling effects of fatigue. The scientists found that the brain needs good-quality sleep immediately after practicing a task if it is to learn to improve at it. Those who substitute study for sleep, particularly those who miss deep or “slow-wave” sleep will get little benefit from their extra effort.

The findings, published in Nature Neuroscience, add to a growing body of evidence that sleep is vital to the learning process. The results, Professor Stickgold said, suggest that a good night’s sleep immediately after learning is “absolutely required” to embed new skills in the memory. “We think that the first night’s sleep starts the process of memory consolidation,” he said. “It seems that memories normally wash out of the brain unless some process nails them down. My suspicion is that sleep is one of those things that nails them down.”

(Adapted from Focus on IELTS by Sue O’Connell)

The word ‘they’ in paragraph 2 refers to ______.