If a civilization as technologically advanced as human civilization existed on another planet and that planet were within 50 light years of Earth, that civilization would have found evidence of intelligent life on Earth and could have easily contacted us. Scientists can thus rule out the possibility of finding a civilization as technologically advanced as our own within 50 light years of Earth. Which one of the following is an assumption required by the argument? ['There is no reason to doubt the possibility that there are technologically advanced civilizations on planets more than 50 light years from Earth.', 'A technologically advanced civilization on another planet would want to communicate with intelligent life that it detected on Earth.', 'Intelligent life forms on other planets would be able to recognize all signs of intelligent life on Earth.', 'Scientists who are searching for evidence of extraterrestrial life forms generally focus their search on evidence of technologically advanced life forms.']