All known techniques for handling complex problems successfully seem to fall into one of the three classes: subdividing the problem (51) , ignoring irrelevant detail in a safe way (52) , and having an independent agent (53) the internal consistency (contextual checking). The first two provide guidelines for solving the problem, the third serves to provide early warnings. A good programming language supports all three.
In subdividing problem, some of the subproblems may be similar to the (54) problem. It leads us to a (55) solution. This (55) solution is viable provided each of the subproblems is easier to solve than the original problem.

A. check B. original C. perform D. recursive

电力网按其在电力系统中的作用不同分为哪。