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The bicycle is a two-wheeled,light-weight machine,which is propelled by human power.It is a efficient,cheap,easily manufactured,and one of the world's most popular forms of transport.The first pedal-driven bicycle was built in Scotland in 1839.Since then the basic design-of a frame,wheels,brakes,handlebars,and saddle-has been gradully improved,with the addition of a chain,gear system,and pneumatic tyres (tyres inflated with air).The recent invention of the mountain bike (all-terrain bike) has been an important development.With its strong ,rugged frame,wide tyres,and 21 gears,a mountain bike enables riders to reach rough and hilly areas that were previously inaccessible to cyclists.
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