单选题
American architecture is (71) its best when it is concerned with (72) that have a practical purpose. Factories, office buildings, public buildings, rail terminal and airports, and other such (73) show American architecture at its (74) imaginative and graceful, as well as its most (75) . The single most important American (76) design is the skyscraper, a style (77) in the late nineteenth century and since (78) as the trademark of American building. (79) the development of the steel skeleton and brick for buildings, the weight and low strength of stone and brick limited the (80) of the city office buildings (81) about twelve floors. (82) American cities grew larger in closing years of the 19th century, the (83) on which the structures might be placed because more valuable, the (84) for taller buildings increased. The use of steel (85) for construction was a direct response (86) this demand. The first office building to use the steel frame (87) was constructed in 1884. It soon became clear (88) . a steel--frame building could rise (89) four times as high us one of brick and stone. In a little more than ten years, the use of steel (90) has revolutionized the design of city office buildings, and the skyscraper was born.
【答案解析】as引导一个时间状语从句时，有“当……时”，也有“随着……”的意思。例如：As a young man. he joined the Eignth Route Army. 他在青年时代参加了八路军。(当……时)as time goes on everything changes. 随着时间的推移，一切都在变。
【答案解析】选项A是“必要性”的意思，如表示“……的必要性”时，通常多用介词of连接，选项B意指“紧急性”， C项则意思是“需要，要求”，D项是指“需要，需求”。我们可以举几个例子：the necessity of smashing the old state machine打碎旧的国家机器的必要性；a matter of urgency紧急事情；a tremendous requirement of this material对这种材料的大量需求；There's a great demand for these commodities. 这些商品的需求量很大。C、D之间的差别就是后面的介词of，for。一般来说，the/a demand for sth. 表示“对某物的需求”，而requirement则用of表示“……的需要”。因此答案应选D。