Attention, coffee lovers! We have for you, the best coffee machine ___ invented.

It’s always difficult to live in a foreign country, ______ when you don’t speak the language. 

John is very lazy. He falls ______ behind among the students.

  -----When will they come back? 　　-----I _____ don’t know.

(2013 ·湖北武汉武昌高三联考)Smiling doesn't ________ mean you're happy.Sometimes, it just means that you're strong enough to face problems.