题干

如图,电源电压为30V,内电阻不计,一个“6V,12W”的电灯与一个绕线电阻为2Ω的电动机M串联接入电路.已知电路中电灯正常发光,则电动机输出的机械功率为(    )

A:40W

B:44W

C:48W

D:60W

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答案(点此获取答案解析)

A

同类题4

   Zhalong is a nature reserve in Heilongjiang in north-east China. It is one of the world’s most important wetlands. The reserve covers an area of 210,000 hectares. The area provides food and shelter for wildlife. It is an ideal home for different kinds of plants, fish and birds.
   Many birds live comfortably in Zhalong Nature Reserve all year round, while some only stay there for a short time. There are many fish in the wetlands, and the birds can easily catch them for food. Zhalong has long been called “the home of the crane”. There are 15 crane species (物种) in the world. 8 of them are in China and there are 6 species in Zhalong area. It is an important living area for the rare red-crowned cranes. There are not many red-crowned cranes in the world. Every spring and summer, red-crowned cranes lay and hatch (孵化) their eggs in Zhalong.
   Some people want to change the wetlands to make more space for farms and buildings. This means there will be less and less space for wildlife. More and more birds are in danger because they do not have enough living space. Many of them died. The Chinese government wants to protect these endangered birds, and they can be safe in Zhalong.
   Every year, a lot of tourists go to Zhalong to watch birds. This year, members of our Bird-watching Club are going to study the different kinds of birds in Zhalong and the changes in their numbers. The study begins next month.
   We _______ once a year. We are now inviting secondary school students to help. We need more people to help us count and do something to help the birds.
   Many people do not understand the importance of the wetlands. We hope this information will help them understand and make them actively take action to protect wildlife.