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

1. A.
   What;from
2. B.
   That;back
3. C.
   Which;to
4. D.
   It;in

____ 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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