题目

在真空中有a、b两个点电荷，b的电荷量是a的3倍，如果a受到的静电力是F，则b受到的静电力是（ ）A. F B. F C. 3F D. 4F 答案：【答案】A【解析】试题分析：电荷对b电荷的静电力与b电荷对a电荷的静电力是一对作用力与反作用力，大小相等．所以b受到的静电力是F．故A正确，B、C、D错误．故选A．They leap from helicopters or speeding boats, bringing aid to swimmers who get into trouble off Italy’s popular beaches. For these dog lifeguards, the doggie paddle （狗刨式游法）does just fine. These “lifedogs” tow a buoy(救生衣) that victims can grab, or a raft they can sit on to be towed back to shore, and unlike their human counterparts(与对方地位相当的人), they can easily jump from helicopters and speeding boats to reach swimmers in trouble. With millions flocking(人群) to Italy’s crowded beaches each summer, the Italian Coast Guard says it rescues about 3,000 people every year — and their dog helpers are credited with saving several lives. It takes three years for the canines to reach expert rescue status, and currently 300 dogs are fully trained for duty, said Roberto Gasbarri, who co-ordinates the Italian School of Canine Lifeguards program at a centre outside of Rome in the seaside town of Civitavecchia. “Dogs are useful in containing the physical fatigue（疲劳） of the lifeguard, to increase the speed at which casualties（遇难者） are retrieved（找到并衔回）, to increase the security of both the casualty and of the lifeguard,” Gasbarri said. The Civitavecchia centre is one of a dozen around the country for the school founded more than 20 years ago in the northern province of Bergamo by Ferruccio Pilenga, whose first trainee was his own Newfoundland. The school will train any breed（品种）, as long as they weigh at least 30 kilograms, but Labradors, Newfoundlands and golden retrievers are most commonly used because of their natural instinct for swimming. Each dog works in tandem with（同 ……合作） a human lifeguard, who also acts as the animal’s trainer. 1.Which of the following can act as the best title fo the passage?   A. Dogs to the Rescue                B. Dogs Are Good at Doggie Paddle   C. The Wonderful Performance of Dogs  D. Dogs: Our Loyal Friends 2.What advantage do these life-dogs have over human lifeguards in a rescue?   A. Dog can swim very fast                      B. Dogs can finish rescue work with ease.   C. Dogs can finish the great work free of charge.    D. Dogs can easily be trained 3. Which of the following is TRUE about the Civitavecchia center according to the text? A. Any breed can be trained there.   B. The ones who have the gift for swimming are welcomed.   C. After a better training, the dogs can work alone.   D. It is the only place for training dog lifeguards in the school. 4.What does the writer want to tell us in the 4th paragraph?   A. Without the dog lifeguards, 3,000 people would lose their lives every year.   B. The buautiful beaches attract millions of people every year.   C. Dog lifeguards play an important part in a rescue.   D. Italy is a famous place of interest.