OG 83

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.

解释看不懂，能不能请牛人解释一下