2.(C) Not having been able to sole the emission problems so far-cannot result in lack of continuity, lack of motive or lack of alternatives or lack of concern, but can be described as lack of success. So, (C) is the only suitable choice even on consideration of the alternatives for the first missing word. Its pair word �failure�, when substituted in the sentence, means that the designers are still continuing their experiments and may yet succeed in finding a solution to the problem. (C) is therefore the answer.