Since the 1930s aircraft manufacturers have tried to build airplanes with frictionless wings, shaped so smoothly and perfectly that the air passing over them would not become turbulent.
wings, shaped so smoothly and perfectly
wings, wings so smooth and so perfectly shaped
wings that are shaped so smooth and perfect
wings, shaped in such a smooth and perfect manner
wings, wings having been shaped smoothly and perfectly so
since后面是一个句子,可是从句不是也 应该是一个句子嘛?为什么5个选项从句都没有谓语动词,还是我理解错了?!?
Since the 1930s 是一个时间状语吧
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