OG(VR)85

[alva0930]

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

對OG的解釋有一部分看不太懂，
(A) There is one leaves it in the final clause without a referent.
(D) ...; the final it has no referent.

為什麼用there is one與its presence會造成the final it has no referent呢？

[dibert8]

(A) there is 是非限定用法,因此 one 不知道指什麼 (one 是 condition 沒錯,但不是前面的那個 condition ,句意脫節了).最後一個 it = one, OG就是這個意思啦!
(D) it 只剩一個名詞給它指代, presence ,但 its presence <> it (his book <> he), 所以沒得指啦!