Is Mary ( ) duty today 应该填什么介词?

Is Mary ( ) duty today 应该填什么介词? is on duty today who is on duty who is ( ) duty today? who is on duty today Who is ___ duty today? is Han Meimei-----duty today? what is on duty today? your duty is to obey . Who is on duty today?为什么用on duty _____lily______?A.Is,on duty B.Dose,on duty working hard is our duty .是duty 还是duties呢 duty How is Mary How is Mary How is Mary How is Mary How is Mary jim is father is（ )night duty today MARY IS HAPPY MARY IS HAPPY怎么样 Is she _____ duty today?填介词, Who is on duty today ?是什么意思 谢谢