• Gram's staining technique classifies bacteria into Gram-positive and Gram-negative based on the cell envelope (cell wall) structure.

• Gram-positive bacteria have a thick peptidoglycan layer, so they retain the crystal violet (Gram) stain and appear purple.

• Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane, so they do not retain the Gram stain after decolorization and take the counterstain (safranin), appearing pink.

Answer: Both A and R are true, and R is the correct explanation of A.