题目解析:
第2段高亮句主干信息:China’s political unification had come under the pressing demand for a coordinated effort in dealing with the flood problem.
A选项主干信息为flooding has continued,与原文主干信息不符,错误;
B选项主干信息为centralized political organization was necessary to deal with vast flooding problems,which still occur in modern times = which has remained in force for the past 2,000 years,正确;
C选项主干信息为the floods have weakend the political unification,错误;
D选项despite让步逻辑在文中未提及,错误。
故选B。