2. 外语
  3. Before the first non-stop flight made in 1949, it ______ necessary for all planes to land for refueling.

Before the first non-stop flight made in 1949, it ______ necessary for all planes to land for refueling.

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问题 Before the first non-stop flight made in 1949, it ______ necessary for all planes to land for refueling.
选项 A、would be
B、has been
C、had been
D、would have been
答案C
解析
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