Fill in the blanks in the following sentences with the words or phrases given . Change the form where necessary. Though great progress has been made in recent years, China is still a ___________ country. (develop) Traffic in the downtown city was completely ________ by the accident. (blockage) Wealth is seldom _________ happiness.(relate... to) Relation between the two countries __________sharply in recent weeks.(deteriorate) Flu usually _______ in the season of spring.(occur)