We're looking for an answer choice that can be supported by information that's in the passage. Take a look at lines 49 to the end of the passage. The sentence starting at line 49 is basically telling us that in case-based systems currently in development, specific criteria for similarity are fixed by the designer, and similarity can be found only by testing for the presence or absence of these predefined factors. But, the sentence starting on line 54 tells us, it still can't solve "the intractable problem of developing a system that can discover for itself" similar factors. This is exactly what (C) tells us - that even if we put in similarity criteria, the systems still have an important shortcoming. Hope this helps!