互动封闭仿真课堂
1、智能交互 2、电子板书式视频教学 3、在线互动问答 4、封闭课堂启发式教学方式
随堂测试、知识梳理、易错点揭示、总结启迪智能错题本
听课中的错题能自动加入错题本，课后可方便复习及导出错题本升级服务
在线答疑、课后同类题练习、讲义下载、短信提醒原则六、-ing形式或to do可作主语或宾语，-ed形式则不可以
例11. ___________ in a heavy traffic jam is quite an unpleasant experience.
A. Caught B. Having caught C. Being caught D. To catch
解析：本题中is为系动词，前面应该作主语，用-ing形式，所以答案为C。
例12 ___twice a year, whether it is a car or a bus or a truck, is the rule that every driver must obey in this city.
A. Examining B. Examined C. Being examined D. Having been examined
解析：本题中is为系动词，前面应该作主语，用-ing形式，所以答案为C。
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