American architecture is (1) its best when it is concerned with (2) that have a practical purpose. Factories, office buildings, public buildings, rail terminal and airports, and other such (3) show American architecture at its (4) imaginative and graceful, as well as its most (5) . The single most important American (6) design is the skyscraper, a style (7) in the late nineteenth century and since (8) as the trademark of American building. (9) the development of the steel skeleton and brick for buildings, the weight and low strength of stone and brick limited the (10) of the city office buildings (11) about twelve floors. (12) American cities grew larger in closing years of the 19th century, the (13) on which the structures might be placed because more valuable, the (14) for taller buildings increased. The use of steel (15) for construction was a direct response (16) this demand. The first office building to use the steel frame (17) was constructed in 1884. It soon became clear (18) . a steel--frame building could rise (19) four times as high us one of brick and stone. In a little more than ten years, the use of steel (20) has revolutionized the design of city office buildings, and the skyscraper was born. 10（）

A. highest
B. higher
C. high
D. height