21) Some animals ........ a part of their body to save themselves from predators.
Explanation: cast off means to remove
22) He sat .... the chair for dinner.
Explanation: 'on' is used to indicate the surface of something
23) Peter was born ...... 12th June in 1988.
Explanation: preposition 'on' is used to specify dates and days
24) She is wearing a gold ring ..... her finger.
Explanation: preposition 'on' is used to indicate a part of the body
25) She goes to the store on Wednesdays as on this day the products are ... sale.
Explanation: 'on' is used to indicate the state of something, e.g. on sale, on duty, etc.