警察和警察机关不是从来就有的,也不是永世长存的,它随着国家的产生而产生,也必将随着国家的消亡而消亡。( )
参考答案:对
It's really difficult to ______ what he is trying to express. [ ]
A. leave out
B. break out
C. put out
D. make out
什么是生态平衡?如何判断生态系统是否平衡?