A < B < C < D doesn't imply that there's some k such that kA>D
Yes it does.
As I read this I knew my answer would be the dust specks. Since then I have been mentally evaluating various methods for deciding on the ethics of the situation and have chosen the one that makes me feel better about the answer I instinctively chose.
I can tell you this though. I reckon I personally would choose max five minutes of torture to stop the dust specks event happening. So if the person threatened with 50yrs of torture was me, I'd choose the dust specks.