At the meeting they discussed three different ________to the study of math.

 There is ________ in his words.We should have atry.(2011·四川)

It was obvious that the Climate Change Conference in Copenhagen______the global environmental problems.

— Tom, are you going swimming with us?— I’d love to, but something important has ______.

Up to now, no one has come up with a（n）       explanation of why dinosaurs died out, and it remains a mystery．