when and where to build another bridge ___ still unknown A.is B has been

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when and where to build another bridge ___ still unknown A.is B has been

when and where to build another bridge ___ still unknown A.is B has been
when and where to build another bridge ___ still unknown A.is B has been

when and where to build another bridge ___ still unknown A.is B has been
when and where to build another bridge __A_ still unknown A.is B has been

b

选is

很明显是B has been .句子意思:在何时何地建另一座桥是未知的
分析:到现在未知,根据句子结构,用现在完成被动时 即has been done