变压器是利用（）原理来改变交流电压的装置。
A. 电磁感应
B. 能量守恒
C. 变频
D. 万有引力
客车、发电车火灾或爆炸内部烧损换修面积达（），既构成中破。
A. 10m²
B. 20m²
C. 30m²
D. 40m²
Which of the following statements is/are true regarding hydraulic pumps in general? （）
A. Variable volumes can be obtained with gear pumps only by variation of the pump drive speed
B. A radial piston pump houses sliding pistons in a stationary cylinder block through which passes a rotating pintle or ported shaft
C. The amount of fluid displaced per revolution of an axial piston rotary pump is dependent on the angle formed between the cylinder block and valve plate
D. All of the above
从事食品批发业务的经营企业销售食品，应当如实记录批发食品的名称、等内容。记录、票据的保存期限不得少于3年。
A. 正确
B. 错误
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