营养性巨幼性红细胞性贫血的病因是（）和叶酸缺乏引起。
参考答案：维生素B12
I don't understand _______ believe him. [ ]
A. why don't you
B. why you not
C. why you don't
D. why not you
瓦楞纸制的包装机器零件用的纸箱
