Mass deaths of birds were not unheard of, but they normally happened unnoticed at sea or in rural areas away from towns. Many birds are weak and die in winter _______ food is scarce.
There was no sign _______ they had died from diseases such as bird flu - the main worry in such cases. Among the theories was _______ a truck had collided(碰撞)with the crowd.
Early this year, thousands of migrant _______ workers cancelled their insurance due to the lack of _______ policy to transfer insurance between difference regions.