Hmmm, that link is interesting. It says Class A - D are cargo and baggage compartments, but that is not what my text book says.
Class A - Cockpit and passenger cabin
Class B - Accessible cargo and baggage areas
Class C - Engines
Class D - Inaccessible compartment
Class E - Accessible cargo compartments on cargo only aircraft
Other than common sense is there anything preventing you from firing the extinguisher on the incorrect engine?