发生重大事故的基层单位，应于事故发生后的＿日内向上级单位提出事故报告。
A.3
B.5
C.7
D.10
参考答案：C
You can hardly imagine the once clear canal _____ so seriously polluted that no fish can live in it. [ ]
A. have become
B. having become
C. to have become
D. to become
若有如下二叉树，则对此二叉树前序遍历的结果为( )。
