It was not until 1911, the first vitamin was identified, people realized the importance of it．
A. when; which
B. when; that
C. that; which
D. that; that
网友回答
A. when; which
(答案→)B. when; that
C. that; which
D. that; that
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。