Emergency alarm signals are provided on tanks in case of the following which one of these alarms will be of major importance on a double hull tankship with empty ballast tanks?

 Emergency alarm signals are provided on tanks in case of the following which one of these alarms will be of major importance on a double hull tankship with empty ballast tanks? 

1) High concentration of toxic or flammable vapors 

2) a co2 dischargee in the engine room or pumproom 
3) when leveling of oxygen in inert gas entering tanks exceeds 5%
4) high level of oil residues in overboard discharge 

Ans_____1) High concentration of toxic or flammable vapors