A、Because they have a smooth surface. B、Because they are very hard. C、Because they are very expensive. D、Because they turn very.

Watches, which are really small clocks, were first made during the sixteenth century. The earliest watches were made of heavy iron, and people wore them hanging from their belts. Little by little, watches were made smaller so that they could be carried more easily. The invention of the machinery for the mass production of these parts of clocks and watches was an important step in making clocks and watches available to the general public. In some watches, small pieces of precious stones, such as rubies or sapphires, are used at the points on which the wheels turn. Since these stones are very hard, they do not wear down very easily under the friction of the moving wheels. Thus a watch utilizing such stones will keep accurate time over a much longer period than other watches.
    Some modem watches can measure very small fractions of time. These watches are useful in timing athletic events. When a button is pressed on these watches, a special second hand begins to move. This type of watch can also be used to count a patient’s pulse. It would be difficult to imagine today’s world without clocks and watches.