All of the following statements regarding bioavailability of a drug are true except:
1. It is the proportion (fraction) of unchanged drug that reaches the systemic circulation.
2. Bioavailability of an orally administered drug can be calculated by comparing the Area Under Curve (0- ) after oral and intravenous (iv) administration.
3.Low oral bioavailability always and necessarily mean poor absorption.
4.Bioavailability can be determined from plasma concentration or urinary excretion data.