20. (A) The quality was lower in the Neolithic period when works of tusks and bones became part of art and craft. (B) The decline in the Neolithic period resulted in works of tusks and bones becoming part of art and craft. (C) Until the Neolithic period, works of tusks and bones had been part of art and craft. (D) By the Neolithic period, works of tusks and bones had become part of art and craft.