Subjecting D-(-)-threose to a Kiliani-Fischer synthesis yields two other epimeric aldopentoses, D-(+)-xylose and D-(-)-lyxose. D-(+)-Xylose can be

Question:

Subjecting D-(-)-threose to a Kiliani-Fischer synthesis yields two other epimeric aldopentoses, D-(+)-xylose and D-(-)-lyxose. D-(+)-Xylose can be oxidized (with nitric acid) to an optically inactive aldaric acid, while similar oxidation of D-(-)-lyxose gives an optically active product. What are the structures of D-(+)-xylose and D-(-)-lyxose?