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 sentences discuss new scientific research on PFAS. They cover what these chemicals are, what the research found about their health effects, and why these findings are important for medicine. The main focus is on the research, its implications, and how it may help identify diseases in the future.

Sentences 2, 1, 4, and 5 follow a logical order. Sentence 2 introduces the new research and its main claim about gene activity and health problems. Sentence 1 gives background on what PFAs are and why they are used. Sentence 4 builds on the research by suggesting PFAS could be linked to more diseases that have not yet been identified. Sentence 5 wraps up by explaining why these findings matter, showing how they help us understand diseases and improve early detection and treatment.

Sentence 3 does not fit well with the others. It is accurate, but it explains why PFAS are called “forever chemicals” because they stay in the environment. None of the other sentences discusses this idea; moreover, statement 3 does not add to the main points about gene activity, disease, or medical impact. So, it feels like extra information that is not directly related. Therefore, the odd sentence out is 3.