治安警察，是指负责维护社会治安秩序，保障公共安全的人民警察。( )
参考答案：对
He hasn't heard from his friend ______ last month. [ ]
A. since
B. by the end of
C. for
D. until
如图所示，唐师傅想用最省力的方法把一个油桶推上台阶，请你在图中画出这个力的示意图．
