In 1995, two scholars in the United States used radio telescopes with a diameter of 26 meters to search for unknown signals from the depths of the vast universe. However, no radio wave source with the characteristics of the received signals has been found so far, and in the direction of these radio waves, There are no specific stars. What does the phrase "No radio wave source with the characteristics of the received signal found" mean? ["I don't know if the received signal has a radio source.", 'I have received a signal but I am not sure of its source.', 'Not sure where the source of the radio wave has received the signal.', 'The signal has been received but the characteristics of the radio source have not been found.']