During the Diagnostic Test Procedure for a Catalyst Efficiency Code

During the diagnostic test procedure for a catalyst efficiency code, a diagnostic scan tool is connected to the DLC with the vehicle at operating temperature, and the data screen displays the precatalyst oxygen sensor continually switching between 200 mV - 800 mV, while the postcatalyst oxygen sensor's output voltage stays relatively steady at 700 mV. Technician A says that the postcatalyst oxygen sensor is fixed rich and should be replaced. Technician B says that the oxygen sensor readings are both normal. Who is correct?

A) A only
B) B only
C) both A and B
D) neither A nor B