在探究植物光合作用，呼吸作用和蒸腾作用活动中，某同学设计以下探究方案，请你用所学知识进行分析．

（1）如甲图，将植物罩上透明塑料袋，放在光下3-4小时，将塑料袋中气体通过①号试管收集后取出试管，将一支带火星的木条伸进①号试管内，结果木条复燃了，由此得出的结论是：①号试管内含较多的______（气体）．

（2）如甲图，若将透明塑料袋换成黑色的，其目的是为了防止植物进行______作用．上述（1）（2）实验中，分别用透明和黑色两种塑料袋是为了设置______，该实验的变量是______．

（3）在实验过程中，塑料袋内壁出现水珠，是叶片进行______作用散失的水分凝聚而成，这些水分是通过茎的______中的导管自下往上运输来的．

（4）如乙图，将植物罩上塑料袋，放在暗处3-4小时．将塑料袋中气体通入2号试管，结果发现澄清石灰水变浑浊．由此得出结论是：植物进行了______作用，释放出______（气体）．

Americans use more water than any other people in the world. If we continue to use water at the rate we do now, we will soon not have enough to meet our needs. In 1980, 40 billion gallons were used each day in the United States. In 1990, 700 billion gallons were used. The average Americans uses almost 90 gallons of water a day for personal use. And much move water is needed to make the things American people like to have. For example, it takes thousands of gallons of water to make one pound of beef for the dinner table. It takes more than 100,000 gallons of water to make a car. If we include these uses of water, the average American uses about 2000 gallons of water a day.

The best title for the passage is ______.

A．Water Use in the United States

B．The Problem of Water Use

C．Water Use in the World

D．Water Use in the Twentieth Century