Question: how many carbon atoms are there in erythrose?
Answer: Figure 10-4. The aldotetroses contain two chiral carbon atoms, each of which may exist in an R- or S- form, giving rise to four stereoisomers. The two erythrose isomers are mirror images (enantiomers), as are the two threose isomers. Each erythrose is a diastereomer of the threoses, and vice versa.