Compound A is a d-aldopentose. When treated with sodium borohydride, compound A is converted into an alditol that exhibits three signals in its 13C NMR spectrum. Compound A undergoes a Kiliani-Fischer synthesis to produce two aldohexoses, compounds B and C. Upon treatment with nitric acid, compound B yields compound D, while compound C yields compound E. Both D and E are optically active aldaric acids.

(a) Draw the structure of compound A.

(b) Draw the structures of compounds D and E, and describe how you might be able to distinguish between these two compounds using 13C NMR spectroscopy.