Why C rather than A?

C - "expressed unambiguously" - seems like a pretty weak explanation of the phrase "fully explicit" in this context. Looking more broadly at the context, the author states: "Self–similarity is built into the construction process by treating segments at each stage the same way as the original segment was treated. Since the rules for getting from one stage to another are fully explicit and always the same, images of successive stages of the process can be generated by computer." We are told that fractals are generated by using one copy and generating new ones ad infinitum. In other words, taking one example and reproducing it. How is that a weaker answer than "expressed unambiguously," which does very little in the specific context of how fractals are generated as described in the passage? Thank you.