题目内容：
Questions 17-20 are based on the following passage．You now have 20 seconds to read Questions 17—20． What’s the main topic of this passage? A．It’s about the tests of a new airliner before its flyin9．
B．It’s about how to protect a new airliner．
C．It’s about how to train a new pilot．
D．It’s about what the airliner engineers should d0．
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