Why Is Carbohydrate Said to Be "Protein Sparing

Why is carbohydrate said to be "protein sparing"?

A) Because if there is not enough protein in the diet, the body cannot digest starch.
B) Because if the diet provides enough carbohydrate for energy, then protein can be used to make new proteins.
C) Because fiber is needed in order to absorb amino acids into the body.
D) Because if the diet provides enough protein, then glucose will be used for energy.