题目

 Everything comes with_______ price; there is no such thing as free lunch in the world.        A. a            B. the          C. an           D. / 答案：A电火花计时器工作电源是________伏（填电压值）的________电源（填“直流”或“交流”），当电源的频率为50Hz时，每隔________秒打一个点．