It was not until 1911 the first vitamin was identified people realized the importance

问题补充：

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

答案：

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19，人们才意识到它的重要性。故B正确。