Passage Three:Questions 21 to 25 are based on the following passage.After the violent earthquake that shook Los Angeles in 1994, earthquake scientists had good news to report: The damage and death toll (死亡人数) could have been much worse. More than 60 people died in this earthquake. By comparison, as earthquake of similar intensity that shook America in 1988 claimed 25,000 victims. Injuries and deaths were relatively less in Los Angeles because the quake occurred at 4:31 a.m. on a holiday, when traffic was light on the city’s highways. In addition, changes made to the construction codes in Los Angeles during the last 20 years have strengthened the city’s buildings and highways, making them more resistant to quakes. Despite the good new, civil engineers aren’t resting on their successes. Pinned to their drawing boards are blueprints (蓝图) for improved quake-resistant buildings. The new designs should offer even greater security to cities where earthquakes often take place. In the past, making structures quake-resistant meant firm yet flexible materials, such as steel and wood, that bend without breaking. Later, people tried to lift a building off its foundation, and insert rubber and steel between the building and its foundation to reduce the impact of ground vibrations. The most recent designs give buildings brains as well as concrete and steel supports. Called smart buildings, the structures respond like living organisms to an earthquake’s vibrations. When the ground shakes and the building tips forward, the computer would force the building to shift in the opposite direction. The new smart structures could be very expensive to build. However, they would save many lives and would be less likely to be damaged during earthquakes. 第21题：One reason why the loss of lives in the Los Angeles earthquake was comparatively low is that ________. A) new computers had been installed in the buildings B) it occurred in the residential areas rather than on the highways C) large numbers of Los Angeles residents had gone for a holiday D) improvements had been made in the construction of buildings and highways