Answer:
Both technicians are right
Step-by-step explanation:
Technician A is right in saying that reading of 250mv from the O2 sensor is indicative of a lean mixture since it fall below the critical level of 450mv for a rich mixture.
Technician B is right by saying the technician should be checking for a leaking injector. This is because a leaking injection will keep dispensing an uncalibrated amount of fuel which will overwhelm the oxygen level. This by definition is a lean mixture.