Question: 1. what is the relationship between d-erythrose and d-threose?
Answer: D-threose is a diastereomer of both D-erythrose and L-erythrose. The definition of diastereomers is simple: if two molecules are stereoisomers (same molecular formula, same connectivity, different arrangement of atoms in space) but are not enantiomers, then they are diastereomers by default.