17. The cell walls of bacteria, fungi, and plant cells and the extracellular matrix of animal cells are all external to the plasma membrane. Which of the following is a characteristic common to all of these extracellular structures? (A) They must block water and small molecules to regulate the exchange of matter and energy with their environment. (B) They must provide a rigid structure that maintains an appropriate ratio of cell surface area to volume. (C) They are constructed of polymers that are synthesized in the cytoplasm and then transported out of the cell. (D) They are composed of a mixture of lipids and nucleotides.