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【单选题】The example of an egg on a skateboard is used to show that （） A．eggs break easily on a moving skateboard B．cars should not be stopped suddenly C．seat belts are important to safety D．driving is a dange...
A.
Cars are lots of fun, but they could also be dangerous. We have to be careful when we drive them or ride in them.
B.
It’s always a good idea to put on your seat belt when you’re in a car. Why Think about this example: You put an egg on a skateboard(滑板) and give it a push. If the skateboard hits a stone, it will stop, but the egg won’t. It will fly through the air, hit the ground and break.
C.
Now, think what would happen if you tied the egg to the skateboard. When the skateboard hits a stone, the egg won’t go flying. It will stay safely on the skateboard.
D.
Volvo, a famous Swedish carmaker, was the first to use seat belts in 1949. Air bags are also very important for car safety, because sometimes a seat belt isn’t enough. If the car is going really fast and runs into something, seat belts could even hurt the people who wear them.
E.
Most newer cars have air bags in front of and next to the seats. When a car hits something, its air bags will come out quickly--in less than one second--to keep the people inside safe.
【单选题】A. B. C. D.
A.
若(1+x)+(1+x)2+…+(1+x)n＝a1(x-1)+2a2(x-1)2+…+nan(x-1)n，则a1+2a2+3a3+…+nan=（）。