Read the passage about prebiotics and rats carefully and answer this vocabulary 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. In the context of the passage, what is "chow"?

Introduction / Context:
This question evaluates understanding of vocabulary in context. The word “chow” can have several informal meanings in English, but here it appears within a scientific description of feeding rats. The reader must infer its meaning from the surrounding text, rather than rely solely on everyday usage.

Given Data / Assumptions:
- The passage states that researchers “fed three-week-old male rats a diet of either standard chow or chow that included prebiotics”. - The rats are part of an experiment involving different diets. - One diet is labelled “standard chow”, the other is chow supplemented with prebiotics. - The question asks: “What is chow?”

Concept / Approach:
Words in scientific passages are often used in specialised or semi-technical ways. In laboratory animal research, “chow” commonly refers to the prepared food mixture or feed given to animals. By examining how the word is used in the sentence, the reader can deduce that “chow” represents the basic food that can be modified by adding prebiotics. It is not the fibre itself, nor a nutrient class, nor a device.

Step-by-Step Solution:
Step 1: Look at the entire sentence: “Researchers fed three-week-old male rats a diet of either standard chow or chow that included prebiotics.” Step 2: Recognise that this sentence contrasts two types of diet given to rats in the experiment. Step 3: “Standard chow” implies a normal, basic food, while “chow that included prebiotics” is a modified version of that food. Step 4: Therefore, “chow” must be something that can be eaten, and specifically, it is the food mixture for the rats. Step 5: Option C, “Rat food”, captures this sense directly and concisely. Step 6: Other options describe fibres, nutrient classes, sleep aids, or tests, which do not fit the way “chow” is used in the sentence.

Verification / Alternative check:
Try to substitute each option into the sentence: “Researchers fed three-week-old male rats a diet of either standard rat food or rat food that included prebiotics” fits naturally. In contrast, “standard type of dietary fibre” or “standard sleep enhancer” do not logically describe a full diet. The context of feeding animals in a controlled experimental setting strongly supports “chow” meaning animal feed.

Why Other Options Are Wrong:
Type of dietary fibre: The passage earlier states that prebiotics are dietary fibres, but “chow” is the entire diet, not just the fibre component. A class of nutrients: This would not be modified with prebiotics in the way described; instead, the whole food is altered. Sleep enhancer: The passage links prebiotics to changes in sleep but does not refer to chow as a direct enhancer. A medical test: Tests such as EEG are used to monitor the rats; “chow” is clearly something fed, not a diagnostic tool.

Common Pitfalls:
Candidates sometimes overthink simple contextual clues and assume every scientific word is highly technical. In fact, many scientific texts use ordinary or colloquial words in slightly specialised ways. Another mistake is to confuse “chow” with “chew” or with specific nutrient terms. The best strategy is to examine how the word functions in the sentence and what role it plays in the described process.

Final Answer:
In this passage, “chow” refers to rat food, that is, the basic feed given to the animals in the experiment.