Although d-galactose contains five stereogenic centers (in its cyclic form) and is optically active, its oxidation with

Question:

Although d-galactose contains five stereogenic centers (in its cyclic form) and is optically active, its oxidation with nitric acid gives an optically inactive dicarboxylic acid (called galactaric or mucic acid). What is the structure of this acid, and why is it optically inactive?