Our playground is not so tidy and beautiful as before students must dirty things on it.A

问题补充：

Our playground is not so tidy and beautiful as before, students must dirty things on it.

A. be stopped to throw

B. be stopped throwing

C. stop to throw

D. stop from throwing

答案：

A. be stopped to throw

(答案→)B. be stopped throwing

C. stop to throw

D. stop from throwing

解析：句意：我们的操场不如以前干净整洁，必须阻止学生乱丢杂物。Stop to do sth停下来去做某事；stop doing sth停止做某事；stop sb from doing sth阻止某人做某事。结合句意必须阻止学生乱丢杂物，而学生students作句子主语，故用被动语态。所以选B。