如图所示是三个质点A、B、C的轨迹，三个质点同时从N点出发，同时到达M点，下列说法正确的是（ ）
①三个质点从N到M的平均速度相同
②三个质点从N到M的平均速率相同
③到达M点时A的瞬时速率最大
④从N到M过程中，A的平均速率最大．
A．①④
B．①②
C．②④
D．①③
三个质点同时从N点出发，同时到达M点，说明运动时间相同
①平均速度等于位移与时间的比值，由于位移与时间都相等，故平均速度相同，故①正确；
②平均速率等于路程与时间的比值，由于时间相等，路程不等，故平均速率不相等，故②错误；
③虽然时间相等，但A到达M点时A的瞬时速率无法比较，故③错误；
②平均速率等于路程与时间的比值，由于时间相等，A的路程最大，故A的平均速率最大，故④正确；
故选A．
How many plastic bags do you use every day? 46 there any chance for us to live without them? What can we do to have 47 plastic pollution? A performance in Hangzhou makes people 48 more about the overuse of plastic bags in our lives. In front of a supermarket in Hangzhou , five volunteers 49 white plastic bags and walked around and through the crowd. The performance 50 people to use more cloth bags 51 plastic ones. Some of the plastic bags worn by the performers were collected from customers in two food markets and 52 were collected by 700 students from a middle school. On that day, when leaving the food markets, customers were told to hand over 53 plastic shopping bags. 54 plastic bags were collected in two hours. “ If we do 55 about it, the earth will be eaten up by white pollution. And 56 will become ‘plastic men’,”said one of the performers. “How can you breathe in 57 plastic bags?” A 58 woman asked one performer.“How can the earth breathe 59 there are too many plastic bags?”the performer answered. Starting in June 2008,plastic bags were 60 given for free in supermarkets,shopping malls or food markets all over the country. People are called on﹙号召﹚to use cloth bags when they go shopping.
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