Why Is the Route of Administration Important to the "First-Pass

Why is the route of administration important to the "first-pass effect"?

A) The route can help determine if the drug is being taken as prescribed and therefore is overcoming the first-pass effect.
B) A drug can be significantly reduced to its inactive metabolite before first entering circulation when administered orally.
C) Drugs administered parenterally are highly subject to the first-pass effect and should therefore be dosed accordingly.
D) Bioavailability of drug can be greatly reduced when the drug is administered by a route other than orally.