The united states failed to join the international community in endorsing the

The Kyoto Protocol.  It is a treaty where those who sign it vow to decrease greenhouse effect emissions based on the evidence that there is global warming and that C02 emissions are the source.  It was signed by U.S. President Bill Clinton but rejected later by President George W. Bush.

answer; i believe that the correct answer is (chronological order);

c, they united with once