Read the passage about prebiotics carefully and answer the following question: 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. According to the passage, how are metabolic by-products released?

Introduction / Context:
This question again tests careful reading of a specific factual detail from the passage about prebiotics and gut health. In the text, the author explains what happens when beneficial gut bacteria consume prebiotic fibre. Understanding the sequence of events is important: digestion of fibre, multiplication of bacteria, improvement in gut health, and release of metabolic by-products.

Given Data / Assumptions:
- The passage clearly states that when beneficial bacteria digest prebiotic fibre, they multiply and release metabolic by-products. - Prebiotic fibre is identified as dietary fibre found in certain foods. - Sleep improvement and stress are also discussed, but they form a different part of the findings. - The question specifically asks about the condition under which metabolic by-products are released.

Concept / Approach:
To answer detail-oriented questions, the candidate should find and quote the part of the passage that describes the process. The relevant sentence links digestion of prebiotic fibre by good bacteria with two results: multiplication of bacteria and release of metabolic by-products. The option that best restates this cause-and-effect relationship is the correct one. It is essential to avoid options that mix this process with the later discussion about sleep and stress, because the passage separates gut events from behavioural outcomes.

Step-by-Step Solution:
Step 1: Locate the sentence discussing metabolic by-products. It reads: “When beneficial bacteria digest prebiotic fibre, they not only multiply, improving overall gut health, but they also release metabolic by-products.” Step 2: Identify the condition or action: “When beneficial bacteria digest prebiotic fibre.” Step 3: Note the result: the bacteria release metabolic by-products as part of this digestion process. Step 4: Recognise that prebiotic fibre has already been described as dietary fibre, so “digest dietary fibres” is a natural restatement. Step 5: Compare with option B: “When good bacteria digest dietary fibres.” This closely matches the text, with “good bacteria” equivalent to “beneficial bacteria” and “dietary fibres” equivalent to “prebiotic fibre”. Step 6: Evaluate other options, which do not match the specific wording of the passage.

Verification / Alternative check:
Consider whether any other option can be supported by the passage. Option A, “When good bacteria help sleep”, is too vague and misplaces the causal link, since the text does not say that helping sleep directly releases by-products. Option C, “When the gut becomes rich in nutrients”, is not specifically stated. Option D, “After recovery from stress due to sleep”, confuses the sequence of events in the study. Option E, “When rats are under experimental stress”, is also not indicated as the cause of by-product release. Only option B is explicitly backed by the passage.

Why Other Options Are Wrong:
When good bacteria help sleep: The passage links by-product release to digestion of fibre, not directly to effects on sleep. When the gut becomes rich in nutrients: This is an inference not directly stated in the text and does not mention digestion of prebiotic fibre. After recovery from stress due to sleep: The study does mention sleep changes after stress but does not say this causes metabolic by-products. When rats are under experimental stress: Stress is part of the design, but the release of by-products is directly tied to digestion of fibre.

Common Pitfalls:
Many students misinterpret cause and effect when several scientific events are described together. It is easy to link metabolic by-products to improvements in sleep or to stress, even though the passage associates them clearly with digestion of fibre. To avoid such confusion, always identify which clause is the cause and which is the effect. Paying attention to words like “when” and “because” helps keep track of causal relationships.

Final Answer:
According to the passage, metabolic by-products are released when good bacteria digest dietary fibres.