Seeing the headmaster entering the class-room, the football was quickly hidden by the students.

• A. The headmaster entering the class-room being seen by the students, they quickly hid the football.
• B. When the students saw the headmaster, they quickly hid the football.
• C. Having seen the headmaster entering the class-room, the football was quickly hidden by the students.
• D. After the headmaster was being seen entering the class-room, the students quickly hid the football.