Which sentence best expresses the main idea?
The human intestine contains trillions of microorganisms, mostly bacteria, which together are called the gut microbiome. Many of them help the body to digest fibre that human enzymes cannot break down, and they produce short-chain fatty acids that nourish the cells lining the gut. Others compete with disease-causing bacteria for space and food. The balance of species changes with diet, age and the use of antibiotics, which may reduce the variety of bacteria for weeks or months. Studies have linked a less varied microbiome with several diseases, but a link alone does not show that the bacteria are the cause; the disease may itself change the microbiome. For this reason, scientists have moved from observing people to experiments in which bacteria are transferred between animals. Such experiments have shown that some traits, such as a tendency to gain weight, can be passed on in mice, but it is not yet clear how far this applies to people.
Answer and explanation
Correct answer: C. It aids digestion and may affect health, though cause and effect are uncertain
- Step 1: The text describes benefits and links with disease. Step 2. It warns that a link does not prove cause. Answer: it helps digestion and may affect health, but cause is uncertain. Why not the others: these claims contradict the text.
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