题目

Millions of Americans are living in areas at high-risk of river flooding, many of whom are completely unaware of the danger.While the risk of coastal flooding from extreme storms and rising seas rightly attracts plenty of attention, Americans are being urged to look inland for a similar threat receiving far less recognition: river flooding. New research conducted by the University of Bristol has shown that as many as 41 million Americans are highly at risk from river floods – more than three times the number previously estimated – with real estate (住宅区) in areas such as Louisiana, Fl如图，在高2米，坡角为30°的楼梯表面铺地毯，地毯的长至少需    米．