D　　Some radio signals were heard in 1967. They were coming from a point in the sky where there was unknown star. They were coming very regularly, too — about once a second, as if they were controlled by clock.　　The scientists who heard the signals did not tell anybody else. They were rather afraid to tell in case they frightened people. The signals were coming from a very small body—no bigger, perhaps than the earth. Was that why no light could be seen from it? Or were the signals coming from a planet that belonged to some other star? There was no end to the questions, but the scientists kept the news secret. "Perhaps there are intelligent beings out there." they thought, "who are trying to send messages to other planets, or to us?" So the news was not given to the newspaper. Instead, the scientists studied the signals and searched for others like them. Well, all that happened in 1967 and 1968. Since then scientists have learnt more about those strange, regular, radio signals. And they have told the story, of course.　　The signals do not come from a planet; they come from a new kind of star called a "pulsar"(脉冲星). Pulsars are strong radio stars. They are the smallest but the heaviest stars we know at present. And we can be sure of this: no intelligent beings are living on them. Until now over a hundred other pulsars have been found, and most of them are very like the first one.　　Recently, Chinese researchers have discovered six pulsars, which are super heavy remains of massive stars, using its Five-hundred-meter Aperture Spherical Radio Telescope, known as FAST, thus opening a "new era of Chinese original space discovery". Li Di, the telescope's deputy chief engineer, said FAST is set to be fully operational by the end of 2019. Meanwhile, scientists there will continue to test FAST and cooperate with foreign scientists on space exploration.The radio signals discussed in this passage _____.A. were regularB. were controlled by a clockC. were heard in 1967 onlyD. were secret messagesThe scientists did not tell people about the signals because _____.A. the signals stood for secret messagesB. people might be frightenedC. the signals stood for unimportant messagesD. people would ask them too many questionsA pulsar _____.A. is a well-known satelliteB. is a small and light starC. has intelligent beings that were known at that timeD. is a small heavy star sending out strong radio signalsWhich of the following is TRUE?A. The radio signals were sent by a sky body which was known at that time.B. Scientists have searched for pulsars for many years but found none.C. Chinese researchers have discovered six pulsars using FAST.D. Now is the new era of Chinese space discovery with perfect FAST.