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Where ___ for your next holiday? （）打点计时器应接在电压为4～6V的直流电源上（2）实验桌上有下列器材可供选用：铁架台、电磁打点计时器、复写纸、纸带、秒表、低压交流电源（带开关）、导线、天平、刻度尺．其中不必要的器材有___，缺少的器材有___．