A Customer Complains That the Heater Works Sometimes, but Sometimes

A customer complains that the heater works sometimes, but sometimes only cold air comes out while Driving. Technician A says that the water pump is defective. Technician B says that the cooling system could be low on coolant. Which technician is correct?

A) Technician A only
B) Technician B only
C) Both technicians A and B
D) Neither technician A nor B