题目内容：
请根据短文内容，回答题。Whether you are an American citizen or a foreign national living in the United States，travel abroad requires preparation．Travel may be for the purpose of attending meetings or activities in other countries，visiting families or simply taking a vacation．This page is intended to provide guidance and useful resources．
What information do U．S．citizens need?
Depending upon the requirements of the country to be visited，citizens of the United States need to carry a passport that is valid for six months beyond the visit or documented evidence of their citizenship and identity(身份)．In addition to a passport，some countries will require a visa．For more information，see Visa and Foreign Entry Requirements．
Current information on passports is available at the Department of State Passport Services and Information website．There is a new passport requirement for U．S．citizens traveling between the United States and the rest of the Western Hemisphere(半球)．For details，see Western Hemisphere Travel Initiative．
When traveling to some specified countries，U．S．citizens may be subject to restrictions on publishing，the exchange of information，the sponsorship of conferences，etc．
What is the usual minimum limit to the validity of the passport held by an American citizen? A．There is no limit to the validity of the passport．
B．The passport must be valid for half a year beyond the visit．
C．The citizen can apply for a passport as long as six months．
D．The minimum limit depends on the nature of the citizen’s visit．
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