再生障碍性贫血的主要原因是
A．造血原料缺乏
B．红细胞内在缺陷
C．红细胞破坏过多
D．骨髓造血功能衰竭
E．红细胞寿命缩短
参考答案：D
---I came to your office yesterday morning, but nobody was in.
--- Sorry, we _______ a meeting .
A．have had
B．are having
C．were having
D．had
用麻花钻钻孔，已知钻床的转速200转/分钟，花1分钟时间，麻花钻进给了4mm，请问此时进给量为（）
A.0.04mm
B.0.12mm
C.0.02mm
