這不是語法(grammar)的問題。這是測試你的邏輯分析能力。

It's 'b' because, although 'c' means the same thing, it's grammatically awkward in the sentence. Whereas, 'b' is pronounced without conflict.

A and B mean the exact same thing... and they are both correct here. Were it not for your help, she couldn't have succeeded. If it were not for your help, she couldn't have succeeded. They are an example of a mixed conditional when the condition refers to a past likely consequence. Ex. If I knew her, I would have kissed her hand. C is the standard conditional sentence referring to a past impossible event...it is also correct. Had it not been for your help, she couldn't have succeeded. D is obviously not correct. So, I agree with Erik. Whoever wrote the multiple choice problem made a mistake. So you were right, because you chose the classic answer and problably the one that TOEFL would require!

In the subjunctive mood, many options are allowable. In the question posed here, the only one that is NOT allowed is D. All of the others are correct. We could add to the list these: "But for your help..." "If not for your help..." I don't understand why the key is preferring B. over the others (A. or C.). In my understanding, these three are all acceptable and proper English grammar. I should point out that this is VERY formal English, whichever you choose. Everyday English would be this: "She couldn't have succeeded without your help." -or- "Without your help, she couldn't have succeeded."