Before the first nonstop flight made in 1949, it_____ necessary for all planes to land for refueling.
A．would beB．has beenC．had beenD．would have been
C

試題分析：考查時態(tài)：句意：在1949年世界上首次不間斷飛行之前, 所有飛機有必要的降落加油。說明“有必要”是在1949年之前，用過去完成時，選C。
點評：時態(tài)的考查主要看上下文的語境和句子的關鍵詞。這題的關鍵是時間狀語Before the first nonstop flight made in 1949。
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