When a historian uses the skill of corroboration he or she

you just have to know. Yanno?

Explanation:

Corroboration is the action of double checking information from one source, and searching for different sources, this is done in several social sciences, like history, sociology, anthroplogy, and in journalism, to chech te veracity of information that we have from a source.

Open Door policy, statement of principles initiated by the United States in 1899 and 1900 for the protection of equal privileges among countries trading with China and in support of Chinese territorial and administrative integrity.

Explanation:

The statement was issued in the form of circular notes dispatched by U.S. Secretary of State John Hay to Great Britain, Germany, France, Italy, Japan, and Russia. The Open Door policy was received with almost universal approval in the United States, and for more than 40 years it was a cornerstone of American foreign policy in East Asia.

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