Read the passage carefully and choose the best answer to each question out of the four alternatives. Prebiotics are the lesser-known gut-health promoters which serve as food for good bacteria inside the gut. "We found that dietary prebiotics can improve non-REM (random eye movement) sleep, as well as REM sleep after a stressful event," said Robert Thompson, a PhD researcher at University of Colorado Boulder in the U.S. Prebiotics are dietary fibres found naturally in foods like artichokes, raw garlic, leeks and onions. When beneficial bacteria digest prebiotic fibre, they not only multiply, improving overall gut health, but they also release metabolic by-products. Researchers fed three-week-old male rats a diet of either standard chow or chow that included prebiotics. They then monitored the rats' body temperature, gut bacteria and sleep-wake cycles ? using electroencephalogram (EEG), or brain activity testing over time. Findings revealed that the rats on the prebiotic diet spent more time in non-rapid-eye-movement (NREM) sleep, which is restful and restorative, than those on the non-prebiotic diet. How are metabolic by-products released?