所有乘客都必须系好安全带。
It is necessary for all passengers to fasten their seat belts.
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it英 /ɪt/|美 /ɪt/
is英 /ɪz/|美 /ɪz/
necessary英 /ˈnesəsəri/|美 /ˈnɛsəˌsɛri/
解释
It 虚主语占位，is necessary 表必要性，for all passengers 指明动作主体，to fasten 为真正主语。
It is necessary for all passengers to fasten their seat belts.
It 虚主语占位，is necessary 表必要性，for all passengers 指明动作主体，to fasten 为真正主语。