二冲程柴油机第一冲程初期进行扫气过程，后期进行（）过程。
参考答案：压缩
At least 24 people _______ in the earthquake in Yingjiang on March 10th.[ ]
A. killed
B. were killed
C. are killed
D. have killed
具有预防龋齿作用的是（）
A.铜
B.砷
C.硒
D.氟
E.铬
