The development of the Semantic Web proceeds in steps, each step building a layer on top of another. The pragmatic justification for this approach is that it is easier to achieve （ ）on small steps, whereas it is much harder to get everyone on board if too much is attempted. Usually there are several research groups moving in different directions; this （ ） of ideas is a major driving force for scientific progress. However，from an engineering perspective there is a need to standardize. So, if most researchers agree on certain issues and disagree on others, it makes sense to fix the point of agreement. This way, even if the more ambitious research efforts should fai1， there wil1 be at least（ ）positive outcomes. Once a （ ）has been established ，many more groups and companies will adopt it，instead of waiting to see which of the alternative research lines will be successful in the end. The nature of the Semantic Web is such that companies and single users must build tools， add content， and use that content. We cannot wait until the full Semantic Web vision materializes-it may take another ten years for it to be realized to its full（请作答此空）(as envisioned today, of course).
A. Area
B. Goal
C. Object
D. Extent