单选题 选出应填入下面一段英语中______内的正确答案。
   All known techniques for handling complex problems successfully seem to fall into one of the three classes: subdividing the problem (1) , ignoring irrelevant detailin a safe way (2) , and having an independent agent (3) 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 (4) problem. It leads us to a (5) solution. This (5) solution is viable provided each of the subproblems is easier to solve than the original problem.