题目内容：
The notion of NP-completeness has provided a(66)mathematical definition for(67)intractability of NP problems. But this measure applies only to worst-case complexity. Being NP-complete does not(68)that a problem is intractable on the average case. Indeed, some NP-complete problems are "(69)on average", though some may not be. Levin initiated the study of average-case intractability, He showed that a bounded tiling problem under a simple distribution is average-case NP-complete. Since then, several additional average-case NP-complete problems have been shown within Levin's(70). This paper is intended to provide a comprehensive survey of average-case NP-complete problems that have been published so far, and the techniques of obtaining these results.A．relaxed
B．rough
C．rigorous
D．feasible
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