对地物边界跟踪，是获取地物形态特征的前提。针对不同地物分布特点，地物边界跟踪方法不同
题型：单项选择题
问题：
对地物边界跟踪，是获取地物形态特征的前提。针对不同地物分布特点，地物边界跟踪方法不同。以下适用于线状物体的跟踪方法是（）
A.以图像像元作为跟踪的落脚点，跟踪点的连线作为地物的界线
B.边界跟踪路径从两个相邻地物边界的像元中间穿过
C.边界由一个或多个弧段构成，弧段是具有方向性的线段，线段由有限个有序的边界点构成
D.用一个像素或几个相邻像素表示地物
题型：单项选择题
第三部分 阅读理解（共15小题；每小题2分，满分30分）
阅读下列短文，从每题所给的四个选项（A、B、C和D）中，选出最佳选项。
A
Fifty years from now the world’s population will be declining, with no end in sight. Unless people’s values change greatly, several centuries from now there could be fewer people living in the entire world than live in the United States today. The big surprise of the past twenty years is that in not one country did fertility (生育能力) stop falling when it reached the replacement rate（出生率）—2.1 children per woman. In Italy, for example, the rate has fallen to 1.2. In Western Europe as a whole and in Japan it is down to 1.5. The evidence now indicates that within fifty years or so world population will peak at about eight billion before starting a fairly rapid decline.
Because in the past two centuries world population has increased from one billion to nearly six billion, many people still fear that it will keep “exploding” until there are too many people for the earth to support. But that is like fearing that your baby will grow to 1,000 pounds because its weight doubles three times in its first seven years. World population was growing by two percent a year in the 1960s; the rate is now down to one percent a year, and if the patterns of the past century don’t change completely, it will head into negative numbers. This view is coming to be widely accepted among population experts, even as the public continues to focus on the threat of uncontrolled population growth.
As long ago as September of 1974 Scientific American published a special issue on population that described what demographers (人口统计学家) had begun calling the “demographic transition” from traditional high rates of birth and death to the low ones of modern society. The experts believed that birth and death rates would be more or less equal in the future, as they had been in the past, keeping total population stable after a level of 10-12 billion people was reached during the transition.
56. Which of the following statement is NOT true according to the passage?
A. The world’s population in the future will be reducing endlessly.
B. When the earth population reaches 8 billion, it will see a rapid decrease.
C. In modern society the birth and death rates will be more or less equal in the future.
D. The public now pay little attention to the threat of uncontrolled population growth.
57. What’s reason for the sharp increase of world population in the past two centuries?
A. B
ecause people fear that the world will explode.
B. Because the world’s replacement rate keeps falling.
C. Because people’s values has greatly changed.
D. The passage doesn't mention it.
58. The expression “demographic transition” (Paragraph 3) probably means _______.
A. high death rate to the low one
B. high birth rate to the low one
C. high rates of birth and death to the low ones
D. low rates of birth and death to the high ones
59. We can learn from the passage that _______.
A. in the near future there will be a rapid decline of the world population
B. the birth and death rates of modern society will be unequal in the future
C. there would be the same population living in the world than it in the US today
D. in Western Europe the replacement rate has declined to a negative number