Upon damage to the central nervous system the ______________.
|  | 
   |  | microglia supply glucose to the healthy neurons; | 
   |  | astrocytes release nitric oxide to eliminate weakened neurons; | 
   |  | microglia phagocytize dying neurons; | 
   |  | microglia release nitric oxide to eliminate weakened neurons; | 
   |  | b, c, and d are all correct alternatives; |