18. (A) over(B) for (C) during (D) under

28. In paragraph 2, why does the author mention “Alzheimer’s disease”? (A) To illustrate a potential brain disorder resulting from irregular sleep timing. (B) To highlight a specific protein that harms brain plasticity. (C) To prove that occasional changes to sleep patterns will cause memory loss. (D) To describe a mental health condition that forces the brain to rest.