绿色开花植物的花的主要功能是[     ]

A．吸引昆虫

B．繁殖后代

C．分泌花蜜

D．产生香味

Americans have a great love for informality and nowhere is this better expressed than in their love for nicknames (昵称，绰号). Upon greeting strangers for the first time they will quickly introduce themselves by their nickname. These are shortened forms of their given name at birth and reflect the casual relationships which exist among friends and coworkers. Family names are hardly ever used in daily situations and the use of one’s father’s last name is saved only for rare and formal occasions.
In many traditional cultures the use of the family name is seen as a sign of respect. For Americans, however, it’s the nickname which creates an immediate intimacy between two people upon which to build a relationship. By speaking to another on a first name basis and using his nickname, walls are immediately torn down and equality between two people is established.
At work bosses will refer to their employees by their nicknames. Should an employer use a worker’s family name, you can be sure that some kind of correction or serious business will follow.
This love for nicknames can create a sense of closeness between people who are otherwise not related. Even the presidents of the United States have borne nicknames. Abraham Lincoln was known as Honest Abe. John F. Kennedy was known as Jack or JFK and William Clinton has no reservations in letting people know he prefers to be called Bill, but those who look upon him with disfavor may call him Slick Willy.
Not only people but cities often carry their own nicknames. Chicago is called the "Windy City" because of the p winds which come down from Canada throughout the year. New York City is considered the "Big Apple" for reasons which are still obscure to most people including many New Yorkers.
Not every nickname, however, is positive and some can be rather rude and insulting. If someone should demonstrate a certain negative characteristic, a label will soon be attached to describe that person in an unpleasant way.
Likewise a person’s physical traits which are very dominant may lead to nicknames but not necessarily to one’s liking or choice. A nickname, therefore, is a way of identifying or describing someone or something for better or worse.

In Paragraph 5, the word "obscure" is closest in meaning to "______".

A. funny
B. sound
C. unclear
D. popular