(C) is incorrect because it only deals with a limited part of the passage, not the overall conclusion. The whole second half of the passage is about how the trial was worthwhile, and this is key to the point that the passage is trying to make. When identifying conclusions, I find it helpful to take a step back and ask myself which parts of the passage support which other parts. In this case, the trial being worthwhile (the second half of the passage) does not support the second sentence about the penalty being ineffective, so the second sentence is not the overall conclusion.
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