Chalk markings can not be easily rubbed off from the black-board if kept for a longer time.What do you conclude by this???
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Chalk markings can not be easily rubbed off from the black-board if kept for a longer time.What do you conclude by this???
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Hello..... Chalk markings react with moisture present in the atmosphere. So they stick to the surface of black board. That's why it can't be rubbed off easily.