Read this excerpt from "The City Without Us": The notion that someday nature could swallow whole something so colossal and concrete as a modern city doesn't slide easily into our imaginations. The sheer titanic presence of a New York City resists efforts to picture it wasting away. The events of September 2001 showed only what human beings with explosive hardware can do, not crude processes like erosion or rot. . . . Nevertheless, the time it would take nature to rid itself of what urbanity has wrought may be less than we might suspect.
( 1 ) Which idea is repeated more than once in the excerpt?
A. The September 11 attacks demonstrate humanity's destructiveness.
B. The time nature would need to reclaim the city is very short.
C. Nature conquering a whole city is hard to imagine.
D. Explosive hardware and erosion and rot are similar processes.