83. Any medical test will sometimes fail to detect a condition when it is present and indicate that there is one when it is not. (A) a condition when it is present and indicate that there is one (B) when a condition is present and indicate that there is one (C) a condition when it is present and indicate that it is present (D) when a condition is present and indicate its presence (E) the presence of a condition when it is there and indicate its presence Only choice C, the best answer, produces a sentence in which every pronoun it refers clearly and logically to the noun condition. In choices A and B, the phrase indicate that there . is one does not grammatically fit with when it is not because it has no referent. Choices B and D are imprecise in saying that a test will fail to detect when a condition is present, since the issue is the presence and not the timing of the condition. Further, its presence in D leaves the it in when it is not without a logical referent: it must refer to condition, not presence. Choice E repeats this error; also, the presence ... when it is there is imprecise and redundant. 句意不明白，不理解句子的意思。 所以解释也看不懂。 求大家帮忙！ | 
