What Allows for Selective Toxicity in a Medication

What allows for selective toxicity in a medication?

A) The medication acts against an essential component or biochemical process of microorganisms that does not exist in human cells.
B) The medication is converted into a non-toxic form by the liver in people, but remains highly toxic in bacteria which cannot process the drug.
C) The medication acts against an essential component or biochemical process of human cells that does not exist in microorganisms.
D) Only some medications cross from the blood into the cerebrospinal fluid of humans.
E) Some medications have a very extended half-life AND only some medications cross from the blood into the cerebrospinal fluid of humans.