1) J.K. Rowling's "Harry Potter" series of books has sold over 500 million copies worldwide and has been translated into 80 languages.

2) The word "triangle" contains four vowels: I, A, E, and E.

3) International relations encompass the study of relationships between nations, including diplomacy, treaties, and foreign policy. It examines the interactions and dynamics among different countries in the global arena.

4) The word "oxygen" contains two vowels: O and E.