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 from throwing
C. stop to throw
D. stop from throwing
答案：
A. be stopped to throw
(答案→)B. be stopped from throwing
C. stop to throw
D. stop from throwing
解析：句意：我们的操场不不以前那样干净漂亮了。同学们必须停止在操场乱扔脏东西。stop sb. from doing sth. 也是阻止某人做某事的意思must be done 必须停止做某事。故选B
