问题
单项选择题
()制动有机械制动和电力制动两种。
A、同步式电动机
B、异步式电动机
C、鼠笼式电动机
D、直流电动机
答案
参考答案：D
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()制动有机械制动和电力制动两种。
A、同步式电动机
B、异步式电动机
C、鼠笼式电动机
D、直流电动机
参考答案：D
Stone is all around us. Stone is usually very hard, it can also be soft. Stones have many uses. In some , artists carve (雕刻) beautiful things out of soft stones, because they can be shaped easily. In ancient times, people carved persons or animals out of soft stones, which could be carried along with. While in some European countries, statues (雕塑) are carved hard stones and they do not change easily. In summer, the hot sun on these statues and in winter the snow falls on them, but they remain beautiful. Stone is strong and long-lasting. So, it is enough for buildings. A house built of stone does not catch fire as easily as made of wood. Some stones are coloured, so they make the look wonderful. People can also get a fire with stones. Besides, some kinds of stones can be used to jewellery (珠宝). We’ve found stones are really amazing.
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