．下列仪器可用来判断导线中的电流方向的是：
A．小灯泡
B．开关
C．小磁针
D．电阻箱
答案：C
A、小灯泡只能粗略的判断电流的有无或大小，但无法判断电流的方向，故该选项错误；
B、开关只能控制电路的通断，无法判断电流的方向，故错误；
C、据电流的磁效应可知，电流所产生的磁场的方向与电流的方向有关，所以据电流旁边小磁针的偏转情况可以判断电流的方向，故正确；
D、电阻箱只能改变接入电路中电阻的大小，但无法判断出电流的方向，故错误．
故选C．
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| Each year a million cows in Africa die from east coast fever.The disease is spread by tick bites.Young cows are most at risk；they can die within days.Farmers and herders can lose up to half or more of their calves to east coast fever. The disease is widespread in eleven countries.And experts say it now threatens ten million more animals in new areas including southern Sudan. Researchers first developed an experimental vaccine against east coast fever thirty years ago.The vaccine works by a process called "infection and treatment"．The animals are infected with whole parasites and treated with antibiotics at the same time.This keeps the disease from developing. Controlling east coast fever has meant a better life in areas that have gotten the vaccine. For example,the vaccine has been available for a group of Maasai herders in northern Tanzania for about seven years.They used to lose threefourths of their newborn calves each year.Now,most survive.As a result,many people have extra cattle to sell,and use the money to pay for school for their children. But making the vaccine more widely available-especially in rural areas-has been difficult. Farmers have been using supplies produced in the nineteen nineties.Recently there was a shortage.The International Livestock Research Institute made one million doses at the request of African officials.But that supply is only temporary.Another problem is that the vaccine must be kept extremely cold. Now,the nonprofit Global Alliance for Livestock Veterinary Medicines is trying to expand production and lower the cost.GALVmed spokesman Hameed Nuru says mobile phones have helped lower some barriers to distribution. HAMEED NURU:"Now,with the advent of cellular technology,most of the people we do reach,such as the Maasai pastoralists,they all have cell phones.And they now call the delivery agent who can now come and meet them at a particular place and do the vaccination for them." The vaccine is not cheap.But Hameed Nuru says the herders get together to sell a bull and use the money to vaccinate all their animals.They understand that they are getting value for their money:A cow is worth nearly twice as much if it is vaccinated. A goal is to have local people develop businesses supplying the vaccine. HAMEED NURU:"People are now seeing that they can actually make a business from supplying this vaccine and getting out to the very rural areas where there is a market for this." The efforts are supported by the Bill and Melinda Gates Foundation and the British government. East Coast Fever
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