重点机械的管理实行五优先，即日常维护和故障排除（）、配件准备、更新改造、（）。
A、维修
B、服务
C、资金
D、承包与核算
参考答案：A, D
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
