【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。