A Patient Is Stabilized on Warfarin Therapy and Being Monitored

A patient is stabilized on warfarin therapy and being monitored using the prothrombin time (PT) followed by calculation of the international normalized ratio (INR) . The formula for calculating the INR is: INR = (PT_patient/PT_normal) ^ISI (where ISI = international sensitivity index)
What is used for the PT_normal?

A) Arithmetic mean PT for the control
B) Geometric mean PT for the reference population
C) Arithmetic mean PT as found in a standard reference textbook
D) Geometric mean PT reported by the manufacturer of the PT reagent being used