Those travelers who do not have one of the single-document options must present both an identification document and a citizenship document.

对于那些没有单一证明文件的旅客，则必须出示身份证及公民证件。

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Recently, American border officials have begun demanding extensive identification and proof of citizenship from Canadians, an unprecedented tightening of what is supposed to be an open frontier.

ECONOMIST: Immigration in Canada