【正确答案】：
A）IT IS AMBIGUOUS. AMBIGUITY IS CAUSED BY THE STRUCTURE.  STOP DRINKING CAN BE UNDERSTOOD AS POLICE STOP DRINKING BY THEMSELVES; OR UNDERSTOOD AS POLICE STOP PEOPLE BY DRINKING. 
B）IMPROVEMENT:
（1）THE POLICE WERE ORDERED TO STOP PEOPLE DRINKING ABOUT MIDNIGHT.
（2）THE POLICE WERE ORDERED TO STOP DRINKING BY THEMSELVES ABOUT MIDNIGHT.
【题目解析】：a）句意不明确。歧义是由结构引起的。戒酒（stop drinking）可以理解为警察自己戒酒；也可以理解为警察通过饮酒来阻止人们。b）改进的句子：（1）警方被命令在午夜左右停止饮酒。（2）警方被命令在午夜左右自行停止饮酒。