When you come into the exhibition hall you can see a large board with a notice on it sayin

问题补充：

When you come into the exhibition hall，you can see a large board with a notice on it，saying visitorsnot to touch the exhibits.

A. will request

B. request

C. are requesting

D. are requested

答案：

A. will request

B. request

C. are requesting

(答案→)D. are requested

解析：“request sb.(not)to do sth.要求某人(不要)做某事”是固定结构,根据题干,应使用被动语态。