It was not until 1911, _______ the first vitamin was identified, _______ people realized the importance of it．A.when; whichB.when; thatC.that; whichD.that; that
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B解析考查定语从句和强调句型。定语从句When the first vitamin was identified修饰先行词1911,因为句子很完整，使用关系副词when.第二空与it was构成强调句型。句意：正是知道第一批维他命被识别出来的1911年，人们才意识到它的重要性。故B正确。