Now the only person who knew that Kai Tzu loved Laura and that Laura loved Kai Tzu was Mrs. Spring Fragrance. The reason for this was that, although the Chin Yuen parents lived in a house furnished in American style, and wore American clothes, yet they religiously observed many Chinese customs, and their ideals of live were the ideals of their Chinese forefathers. Therefore, they had betrothed their daughter, Laura, at the age of fifteen, to the eldest son of the Chinese Government schoolteacher in San Francisco. The time for the consummation of the betrothal was approaching. How does paragraph 4 shape the theme of the passage?