题目内容：
根据听到的内容，回答题
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.
参考答案：
答案解析：