历史文化街区内文物古迹和历史建筑的占地面积宜达到保护区内建筑总用地的（）

A.40%

B.50%

C.60%

D.70%

VRML is a （71） for 3D multimedia and shared virtual worlds on the WWW. In comparison to HTML, VRML adds the next level of interaction, structured graphics, and extra（72） (z and time) to the presentation of documents. The applications of VRML are （73）, ranging from simple business graphics to entertaining WWW page graphics,manufacturing, scientific, entertainment, and educational applications, and 3D shared virtual worlds and communities.X3D is the name under which the development of VRML is continued. X3D is based on XML and is backwards （74） with VRML. Furthermore, it is componentized, profiled, and extensible, which makes it possible to use X3D in very different （75）, from high-end visualizations to lightweight applications.

A．link

B．format

C．structure

D．procedure