used to, used for or used as?

Tasers ____________________ ____________________ ____________________ incapacitate suspects without serious injury .
Personal identification Numbers ____________________ ____________________ ____________________ protect cards from criminal use .
Tagging ____________________ ____________________ ____________________ monitor convicted criminals .
Anti - virus software ____________________ ____________________ ____________________ protect computers .
Face - recognition devices ____________________ ____________________ ____________________ admit only the right people .
A helmet ____________________ ____________________ ____________________ head protection .
Batons ____________________ ____________________ ____________________ keep people at a safe distance .
A GPS ____________________ ____________________ ____________________ help people know exactly where they are .
Torches ____________________ ____________________ ____________________ signalling devices .
Luggage X - ray equipment ____________________ ____________________ ____________________ help protect air travellers .