麻烦大佬帮我看看题目选c之后的整个句子是啥意思吧！万分感谢！！！
ac纠结了好久不知道怎么判断…

ab：there be句型，缺少主语；de：when it is not修饰indicate，应该如c修饰it is present

In this sentence the pronoun it does not always refer to condition, but, for the sake
of clarity and correctness, it should. Detect a condition when it is present is correct
and should be followed by a corresponding construction: indicate that it is present
when it is not. The construction of there is one leaves it without a referent.

A There is one leaves it in the final clause without a referent.
B Detect a condition is more precise because it emphasizes the condition
itself; there is one leaves it without a referent

C Correct. The three uses of the pronoun it refer clearly to condition, and
the two parts of the sentence correspond.
D When a condition is present emphasizes the time of the detection rather
than the condition itself. The final it has no referent.
E Presence illogically repeats the same idea as when it is there. The final it
has no referent.

Fail to detect a condition when 和 fail to indicate that 并列