Hebb Identifies Two Structures, Cell Assemblies and Phase Sequences, Which

Hebb identifies two structures, cell assemblies and phase sequences, which play important roles in his learning theory. Which of the following is true of these structures?

A) Cell assemblies are comprised of numerous phase sequences.
B) Cell assemblies are neurological while phase sequences are hypothetical.
C) Cell assemblies and phase sequences are both hypothetical constructs and may not exist.
D) Cell assemblies and phase sequences both exist in the muscular system.