一运动员正在进行长跑锻练，从他的大腿肌细胞中检测到3种化学物质，其浓度变化如下图。图中P、Q、R三曲线依次代表 (    )

    Sunday is more like Monday than it used to be. Places of business that used to keep daytime “business hours” are now open late into the night. And on the Internet, the hour of the day and the day of the week have become irrelevant. A half century ago in the United states, most people experienced strong and precise dividing lines between days of rest and days of work, school time and summer time. Today the dividing lines are still there, but they seem not clear.
    The law in almost all states used to require stores to close on Sunday; in most, it no longer does. It used to keep the schools open in all seasons except summer, in most, it still does. And whether the work week should strengthen its legal（法律的）limits, or whether it should become more changeable, is often debated（争论）. How should we, as a society, organize our time? Should we go even further in relaxing the dividing lines of time until we live in a world in which every minute is much like every other?
    These are not easy questions even to ask. Part of the difficulty is that we seldom recognize the “law of time” even when we meet it face to face. We know as children that we have to go to  school a certain number of hours, a certain number of days, a certain number of years – but unless we meet the truant officer（学监）, we may well think that we should go to school because of social custom and parents’ requirement rather than to the law. As adults we know “extra pay for overtime working” very well, but less familiar with the fact that what constitutes（构成）“overtime” is a matter of legal thing. When we turn the clock forward to start daylight – saving time, have we ever thought to ourselves: “Here is the law in action”? As we shall see, there is a lot of law that has great influence on how organize and use time: compulsory education law, overtime law, and daylight-saving law – as well as law about Sunday closing, holidays, being late to work, time zones, and so on. Once we begin to look for it, we will have no trouble finding a law of time to examine and assess.

阅读下面短文，从每小题所给的A、B、C、D四个选项中选出最佳选项。

    How many calories (卡路里) are there in every meal you eat? That seems like a difficult question to answer. But students at Aitkin High School in the United States can easily answer it now. They put what they eat into an online calorie counter (计算器). It quickly tells how many calories there are in their meals.

In fact, this is part of the school's health class. It is about heart health, nutrition (营养), and fitness. Students can learn basic knowledge about fats, proteins (蛋白质), calories and sugar. Students can also learn how to balance calories in and out. They will know what happens when the balance isn't there. "The online calorie counter can help you make healthy choices. Making one healthy eating decision can really make a difference," Adam Carlson, the teacher of the class, told local newspaper Aitkin Independent Age.

They also talk about good ways to burn calories in the gym. According to Carlson, the gym is a great place to burn calories and get stronger. But it doesn't work for everyone. So they try to find different ways to exercise. "I'm trying hard to teach the students that doing exercise can be fun but hard, and doesn't need to take hours out of the day, but you should keep on doing it for a long time," she said.

    The class has changed students. "I am now watching what I am eating and pay attention to the food labels (标签)," said seventh-grader Karena Burgstaler. Another student James Erickson began to pay attention to how much sugar he eats because it was almost over his daily calorie intake goal.