Con a:
A flight attendant works on planes.
A dancer dances ballet.
A barman works in a bar.
A fireman extinguishes fires.
A botanist works with plants and trees.
A cashier works in a bank.
A waiter works in a restaurant.
A cameraman works on TV programs.
A singer sings in a theatre.
A butcher sells meat. 
Con an:An astronaut travels in a space ship.
An athlete runs on the track.
An auditor audits the books.
An electrician works with electricity.
An author writes books.
An engineer builds bridges.
An announcer works on the radio.
An artist paints pictures.
I have an apple