You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.
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Question: Category: WORLD CAPITALS 
Clue: This city was once known as Dong Kinh & to Europeans as Tonquin

Answer: hanoi, vietnam


Question: Category: HISTORIC BOOKS 
Clue: It was written in 1924 in the fortress of Landsberg am Lech

Answer: mein kampf by adolf hitler


Question: Category: 19th CENTURY AMERICA 
Clue: Graders, gaugers, spikers & bolters built these

Answer:
railroads