Final answer:
Both Technician A's suggestion of a faulty heater control valve and Technician B's suggestion of a problematic cooling system's thermostat could cause an air conditioning system to blow only warm air when on high heat output.
Step-by-step explanation:
The correct answer is C, both Technician A and Technician B are correct. A faulty heater control valve could prevent hot coolant from entering the heater core, which would result in warm air being blown when the heater is on high output. Similarly, if the cooling system's thermostat is stuck open, the engine may not reach its normal operating temperature, and thus the air conditioning system might only blow warm air rather than hot air. Both components play a crucial role in the heating mode of an air conditioning system.