I ran across a car crash in Hong Kong, and ________, both sides just politely exchanged the telephone numbers and cards, a surprise to me.

 He studies the subject _____. 

As is known to all language learners, newly learnt words will soon be forgotten unless ______ used in everyday communication.

 --- You've finally figured out the puzzle！--- Yeah, in a complex way, _______.

The audience are _______young people who like pop music at the concert.