This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General pharmacology, Subject name: Pharmacology
What does the term "bioavailability" mean? 
Select and explain. 
A. Plasma protein binding degree of substance, B. Permeability through the brain-blood barrier, C. Fraction of an uncharged drug reaching the systemic circulation following any route administration, D. Amount of a substance in urine relative to the initial doze
C: Fraction of an uncharged drug reaching the systemic circulation following any route administration
Answer: C: Fraction of an uncharged drug reaching the systemic circulation following any route administration 
Explanation: Bioavailability is defined as the fraction of unchanged drug reaching the systemic circulation following administration by any routeBasic and clinical pharmacology Katzung 13th edition pg 47