I found this one very tricky.

As for AC A, I think I understand that AC A meets the target by introducing a third party, stress, that could be responsible for causing both snoring and smoking. However, I am wondering that what if stress indeed causes both snoring and smoking, and at the same time, smoking causes snoring. A third party cannot exclude the possibility that smoking doesn't cause snoring.

As for AC D, I understand that it indeed doesn't infer any hard causation. But I think it is a quite soft one. The more cases are, the better chance that smoking causes snoring. I think in scientific research, it is indeed has such inference?

Could someone help me with this confusion? Thank you!

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