Vincent Teo (a) It is t heir duty is to clean / cleaning their classroom. /They are responsible for cleaning their classroom. They are on duty cleaning the classroom.
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Vincent Teo
(a) It is their duty is to clean/ cleaningtheir classroom./They are responsible for cleaning their classroom.They are on duty cleaning the classroom.Before duty, they are cleaning the classroom.
(b) They are riding
Vincent TeoCan I say,Yes, you can say that. You don't need 'on' in this case.
(a) They are riding the elephant. (without "on")