摘要： is known that thousands of Chinese worked in the gold mines the late 19th century. A. What; from B. That; back C. Which; to D. It; in

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is known that thousands of Chinese worked in the gold mines ___ the late 19th century.

____ is known that thousands of Chinese worked in the gold mines _______ the late 19th century.

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____ is known that thousands of Chinese worked in the gold mines the late 19th century.

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____ is known that thousands of Chinese worked in the gold mines _______ the late 19th century.