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 flying．
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 do．
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