Examine the Fischer projections of the monosaccharides shown below: 

What is their relationship? Are they structural isomers, diastereomers, enantiomers, or epimers? Additionally, classify each as either a D- or L-isomer.

• A. They are structural isomers. They are both L-isomers.
• B. They are diastereomers. Monosaccharide A is a D-isomer, while monosaccharide B is an L-isomer.
• C. They are enantiomers. Monosaccharide A is the D-isomer, while monosaccharide B is the L-isomer.
• D. They are epimers and they are both D-isomers.

Answer: (D)