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 非公開的;秘密的
Sorry that I cannot tell you. There is a closed-door policy for the investigation.
對不起我不能告訴你 。我們對該調(diào)查采取保密的原則 。
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.
我改變主意的主要原因就是這個 。