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

这个题目要从并列入手，从划线的地方是没有突破点的。when it is not这个省略结构表示前面的主句肯定有it is ....这个结构。有的只有C,这个题目什么没考，就考了一个平行结构。我觉得语法考到最后就是考眼神了