2019-2020学年度人教版必修二Unit3ComputersPeriod1Warming up课件(29张)
2019-2020学年度人教版必修二Unit3ComputersPeriod1Warming up课件(29张)第3页

An abacus is an ancient tool used for counting. The abacus has been in existence in China since the second century BC until now, although the oldest counting boards have been found in fourth century BC Rome.