The contrast-sensitivity of knowledge ascriptions

Knowledge ascriptions are contrast-sensitive. One natural explanation for this is that the knowledge relation is contrastive (s knows that p rather than q). But can the binary view of knowledge (s knows that p) explain contrast-sensitivity? I review some of the linguistic data supporting contrast-sensitivity, and critique the three main binary explanations for contrast-sensitivity. I conclude that the contrast-sensitivity of knowledge ascriptions shows that knowledge is a contrastive relation.