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1.At a private, closed-door meeting in the Vatican on Saturday, the pontiff urged the oil companies to switch to renewable energies for the sake of our planet.
closed-door 非公开的；秘密的
Sorry that I cannot tell you. There is a closed-door policy for the investigation.
对不起我不能告诉你。我们对该调查采取保密的原则。
2.The energy question has become one of the principal challenges...facing the international community.
principal 主要的
The principal reason for my change of mind is this.
我改变主意的主要原因就是这个。
