If you're asking for main idea, the main idea or thesis statement sentence is sentence 1, where they explain what the passage is about.
Answer: the same letter or sound at the beginning of adjacent or closely connected words (i.e. <u>s</u>wift <u>s</u>eagulls <u>s</u>leep <u>s</u>oundly)
The purpose of persuasive writing is to persuade the reader into agreeing to something or agreeing to do something.