Sure thing. First, let's take a look at (B). If (B) is correct, and villagers who performed services for each other issued what we would basically call an IOU, then the envelopes could just be those IOUs instead of what Schmandt-Besserat thinks they are. As such, (B) calls into question Schmandt-Besserat's theory by putting forward an alternative one that fits with the customs of the time.
(C), on the other hand, is irrelevant because it refers to the tablets after 3000 BC, which doesn't change what the tokens on the envelopes meant before that time. (C) refers to a completely different system of recording. Hope this helps.