[单项选择]关于混凝土的技术性质，以下说法正确的是（　）。
A. 混凝土拌合物应具有较好的流淌性 
B. 评价混凝土强度采用混凝土最终达到的值为准 
C. 混凝土抗渗性是影响其耐久性的重要指标 
D. 混凝土的配合比不影响其经济合理性

D Fast food has been a mainstay of American eating for decades. It may have reached a plateau in the United States as the maturing baby-boom generation looks for a more varied menu. Fast food still represents a ＄102 billion-a-year industry, but growth has turned sluggish recently amid tough competition from retail food stores and a more affluent population willing to try new things and spend more, analysts say. Signs of trouble in fast food include price-cutting by industry leaders, including efforts by McDonald’s to attract customers with a 55-cent hamburger, and major players pulling out or selling. O’Pepsico, for example, is selling its last-food restaurant division that includes Taco Bell, Pizza Hut and KFC. "It’s becoming harder and harder for these firms to grow," said Jim Brown, a professor of marketing at Virginia Tech University. "I think in the United States fast food has reached a saturation point because of the number of competitors
A. US fast food industry is growing rapidly.
B. US fast food industry is now in the slow lane.
C. McDonald’s plays an important part in American fast food industry.
D. McDonald’s replaced the leader in time to regain its lost market share.