文献综述难点在于“综”，因为必须根据所查阅大量的文献进行综合的归类、提炼、概括，因此，难以掌握。
参考答案：错
who had arrested him three times for drug-taking.
A．Before George stood the policeman
B．Before George the policeman stood
C．Before the policeman stood George
D．Stood before the policeman George
简述SIEI变频器当前故障和历史故障查询步骤。
