[originaltext] People in the U.S., when they attain legal marriage age and me

People in the U.S., when they attain legal marriage age and meet certain medical requirements, 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 settle an important agreement. When the wedding day is decided upon, the girl sends out wedding announcements or invitations to friends and relatives. They then send wedding gifts to the girl’s home.
   On the wedding day it is supposed to be bad luck for the bride and groom to see each other before the wedding. Another old custom 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’.
   Before the wedding day the groom always chooses a "best man", a good friend to help him and stand beside him during the wedding ceremony. The custom of having a "best man" is thought to have come from ancient times when a strong friend helped the groom and bride escape from the bride’s father.
   When a couple marries, the groom gives his bride a wedding ring. Many marriages are double-ring ceremonies—that is, the bride and the groom exchange rings. The wedding ring is customarily a simple plain gold band. The roundness of the ring symbolizes eternity and announces that the couple is united for life. The wedding ring is worn on the third finger of the left hand. People believe that a vein from the third finger runs directly to the heart.
   Near the end of the reception, which is offered by the bride’s parents, the bride throws her bouquet of flowers to the unwed bridesmaids. The lucky girl who catches it is supposed to be the next in the group to be married. As the bride and groom leave for their honeymoon, the guests all throw confetti on them. This is a symbol of joy and happiness.
Questions:
15.Which of the following requirements is essential if a young couple wants to get married?
16.Why shouldn’t the bride and groom see each other before the wedding on the wedding day according to the passage?
17.What does the custom of having a ’best man’ for the groom tell us about an cient times?
18.Why is the wedding ring worn on the third finger of the left hand?