Five jumbled sentences (labelled 1, 2, 3, 4, and 5), related to a topic, are given below. Four of them can be put together to form a coherent paragraph. Identify the odd sentence out and key in the number of that sentence as your answer.

1.Pfas are a class of about 15,000 compounds most frequently used to make products water-, stain- and grease-resistant.

2.New research suggests exposure to some common perfluoroalkyl and polyfluoroalkyl substances (Pfas) cause changes to gene activity and that these changes are linked to health problems including multiple cancers, neurological disorders and autoimmune disease.

3.These Pfas compounds are dubbed “forever chemicals” because they do not naturally break down in the environment.

4.The research may also point toward other diseases potentially caused by Pfas that have not yet been identified.

5.The findings are a major step toward determining the mechanism by which the chemicals cause disease and could help doctors identify, detect and treat health problems for those exposed to Pfas before the issues advance.

The correct answer is 3.

To identify the odd sentence out, we need to find the common theme that connects four of the sentences to form a coherent paragraph. Let's analyze the core subject of the given sentences.

Step 1: Identify the main theme of the sentences
When reading through the sentences, we can see a clear storyline developing around medical research related to PFAS (perfluoroalkyl and polyfluoroalkyl substances). Sentences 2, 4, and 5 repeatedly reference a specific scientific study focusing on the health impacts of these chemicals. Sentence 2 introduces "New research" linking PFAS exposure to gene activity changes and diseases like cancer. Sentence 4 continues this thought by stating "The research" might point to other unidentified diseases. Sentence 5 elaborates on "The findings" of this research, explaining how it helps doctors treat and detect these health problems.

Step 2: Determine how the remaining sentences fit
We have established that sentences 2, 4, and 5 form the core of a paragraph discussing recent health research on PFAS. Sentence 1 serves as an introductory or background statement for this paragraph, providing a basic definition of what PFAS are and what they are used for (water-, stain- and grease-resistant products). Together, sentences 1, 2, 4, and 5 create a complete, focused paragraph about what PFAS are and the recent medical discoveries regarding their impact on human health.

Step 3: Analyze the odd sentence out
Sentence 3 states: "These Pfas compounds are dubbed 'forever chemicals' because they do not naturally break down in the environment." While this is a factual statement about PFAS, its focus is entirely on environmental persistence. The rest of the paragraph is strictly focused on the biological mechanisms of how these chemicals affect human health and the medical research surrounding it. By introducing an unrelated topic about environmental breakdown, sentence 3 disrupts the unified theme of health and medical research.

Therefore, because it digresses from the core medical and health-related theme of the other sentences, sentence 3 is the odd one out.