A、To follow the ancient custom. B、To make an agreement on loyalty. C、To express his love. D、To make the girl happy. B

From the ancient times, marriage has been an important part of man’s legal and religious systems.
    In addition to the laws governing marriage that each nation establishes, marriage customs have grown up to vary widely from country to country. Anthropologists have studied and written about the numerous marriage ceremonies and customs that exist throughout the world. Let’s see some of the customs and the reasons for their existence in the United States. In the United States, once they meet certain legal age and medical requirements, people are free to choose their own mates. Once a couple has decided to get married, the man customarily gives the girl a diamond ring. The use of a ring comes from the ancient custom of using a ring to seal an important agreement that they will be loyal to each other forever. When the wedding day is decided upon, the girl sends wedding announcements or invitations. Friends then send wedding gifts to the girl’s home. On the wedding day, it’s supposed to be bad luck for the bride and groom to see each other before the wedding. Another old tradition that many people believe will bring good luck to the marriage is for the bride to wear "something old, something new, something borrowed and something blue".
Questions 16 to 18 are based on the passage you have just heard.
16. What do we know about marriage according to the passage?
17. Why does a man give a diamond ring to a woman when they decide to get married?
18. What shouldn’t a bride do before the wedding in the United States?