b - Technician B is correct because a restricted return line in the fuel supply system could cause a continuous high oxygen (O2) sensor signal, which is from a Rich Exhaust or low oxygen content in the exhaust resulting in high HC and CO. Technician A is wrong because low fuel pressure will not cause HIGH CO (Carbon Monoxide) and HC (Hydrocarbons) emissions, rather it could cause low CO (Carbon Monoxide) and HC (Hydrocarbons) emissions.