Australian Foreign Minister Bob Carr said the Boston Marathon bombing is a warning to Australia that the threat of terrorism is constant.It appears none of the 150 Australians who ran in the US city event were hurt when two explosions tore through the finish line, killing three people and injuring more than 100.US authorities are still trying to determine the motive for the attack, which was being treated as an act of terrorism.\"What happened in Boston is a warning,\" Carr told Sky News on Tuesday.\"This is a threat we live with,\" he said