Sara enjoys amusing her friends by________stories.

The environmental problems in China may worsen if __________ unsolved.

There were so many people on the platform waiting to ____ the train that we gave it up. 

 Thanks to the joint efforts made by the leaders of China and the US over recent years, mutually _______ Sino-US relations have made remarkable progress.

A few years after his graduation from a university, the man acquired the    of being a philosopher.