A、They are necessary for modern man. B、They are unnecessary for survival. C、They are as good as computers. D、They are dangerous

Sometimes the simplest of tools is the most useful. Consider, for example, the ruler. A ruler is mainly used to measure and draw straight lines on flat surfaces. It does not have a great many uses, especially since parents and teachers have given up trying to instruct children through spankings.  Some rulers are clear and can be seen through, some made of metal, and some provide both metric and American measurements.  People carry them ill their shirt pockets, briefcases, or purses, to have them available for use any time.
    Of course, the ruler is not only simple but very useful device. Consider, for instance, the pencil, the doorknobs and hinges, and glass. Jet planes and computers are fine, but not necessary for survival. Who, on the other hand, could survive without the simple tools mentioned here?