Which of the following statements is true?

a. The cell walls of bacteria are composed of peptidoglycan.

b. Peptidoglycan is a fatty acid.

c. Gram-positive bacterial walls have a relatively thin layer of peptidoglycan anchored to the cytoplasmic membrane by teichoic acids.

d. Peptidoglycan is found mainly in the cell walls of fungi, algae, and plants.