A number of brain structures are listed below. If an area is primarily gray matter, write a in the answer blank; if mostly white matter, respond with b.  ________    (1) cerebral cortex  ________    (2) corpus callosum and corona radiata  ________    (3) red nucleus  ________    (4) medial and lateral nuclear groups  ________    (5) medial lemniscus  ________    (6) cranial nerve nuclei  ________    (7) spinothalamic tract  ________    (8) fornix  ________    (9) cingulate and precentral gyri