I dont understand this homework question: Aldose A is optically active, but treatment with NaBH4 gives alditol B, which is optically inactive. Aldose...

I dont understand this homework question:

Aldose A is optically active, but treatment with NaBH4 gives alditol B, which is optically inactive. Aldose A undergoes Ruff degradation to give Compound C. When Compound C is treated with NaBH4 , optically inactive alditol D is formed. However, when C undergoes Ruff degradation, optically active D-glyceraldehyde is obtained. Give the structures and names of sugars A-D.