Poll: Logical implication in action

Superseded - see http://news.ycombinator.com/item?id=2125005 instead. ========= In the light of a recent article (http://news.ycombinator.com/item?id=2121428) by Tim Gowers about the difficulties some people have with mathematical logic, I wondered what people here would answer to this question. Suppose I have four cards. Each card has a letter on one side, and a digit on the other. I arrange them so that you can see "D", "3", "K" and "4". I then claim that every "D" has a "4" on the other side. Which cards do you need to turn over to see if I'm right?