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
关于it指代的问题，看了解释还是糊涂，请高人指点。这种类型的题目该如何思考。
谢 | 
