题目内容：
阅读理解In the United Stales，it is customary to exchange gifts on special occasions with family members，friends，relatives，teachers，colleagues，and other associates. But offering gifts to any law enforcement agents（ policeman，slate trooper，etc.），politicians，government employees，or other public employees for favors is considered a serious crime（ known as bribery） in the United States and is punishable by law.
Some newcomers come from countries where it is acceptable to give gifts to somebody in authority in return for a favor. But in the United States，one should never offer any public official a gift for doing something for them，nor should one offer to do a favor in return for help received. Obviously，bribery takes place in the United States, but even so，it is considered illegal.
It is acceptable to give small gifts of appreciations to people who pay for services or who have already rendered services without pay（but who are not public officials），Christmas is a good time to do this. For instance，school children often give their teachers small gifts at Christmas lime，and employers and employees often exchange gifts. Sometimes only the employers or supervisors will give gifts to their staff members without expecting gifts in return.
(1). Americans exchange gifts with their friends______.( )(本题1.5分)
A、 at any time
B、 every week
C、 on some particular occasions
D、 only on holidays
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