19.
(A) Already in the Paleolithic Age, tens of thousands of years ago, the ancient Chinese knew how to make simple tools and ornaments from animal bones and tusks.
(B) Already in the Paleolithic Age, millions of years ago, the ancient Chinese knew how to make simple tools and ornaments from animal bones and tusks.
(C) By the Paleolithic Age, tens of thousands of years ago, the ancient Chinese were known how to make simple tools and ornaments from animal bones and tusks.
(D) By the Paleolithic Age, millions of years ago, the ancient Chinese were known to have been making simple tools and ornaments from animal bones and tusks.