机械呼吸阀应安装在拱顶油罐的顶部（）位置。
A.中央
B.边缘
C.侧面
D.中央与边缘之间
参考答案：A
"_____ you've made! But you should work still harder," Dad said to me.[ ]
A. What big mistake
B. How big mistakes
C. What great progress
D. How great progress
血流速度最慢的是（）
A．主动脉
B．微动脉
C．毛细血管
D．微静脉
E．腔静脉
great progress