A、The signals came from a "pulsar". B、No answer came from a "pulsar". C、Only one pulsar exists. D、Something like a "pulsar" exis

Human beings have never ceased their exploration of the universe. In fact, the cases of witnessing UFO landing or even a direct contact with extraterrestrials have always drawn great public attention.
    In 1967, a group of scientists noticed strange radio signals from space. When the scientists first heard them, they did not tell the newspapers anything about the matter, in case the public were frightened. People are always afraid of strange things from space and even the scientists could not understand what these signals were.
    At first, some of the scientists suggested that the signals were coming from intelligent beings on another planet They thought that these beings were trying to contact people on the earth, or intelligent beings on other planet. However, since there was no star in that part of the sky, there could not be a planet either.
    The scientists studied the signal carefully, and tried to find others like them.
    Since that time, the scientists have found answers to some of their questions. They have discovered that the signals came from a "pulsar"(脉冲星), and they have found at least sixty other pulsars, like the first one.
Questions 15 to 17 are based on the passage you have just heard.
15. What did a group of scientists do when they noticed strange radio signals from space?
16. What did the scientists suggest at first?
17. What have the scientists found since that time?