Answer:
a. it is normally free of microbes
Explanation:
An axenic system is free of microbes. Central nervous system is made of brain and spinal cord. Brain has three parts: brain stem, cerebrum and cerebellum. CNS is an axenic system hence normally it is free of any microbiota. Pathogens may enter CNS by crossing the blood brain barrier by destroying the cells of meninges. Sometimes cerebrospinal fluid can also carry microbes.
