题目

--What happened to you this morning? -- The teacher asked me for my _____ when I was late again. A. meaning            B. idea           C. excuse                   D. answer 答案：C 某机床上使用的一台直流电动机标有“36 V　24 W”字样，测得此电动机线圈的电阻为2 Ω，求此电动机在正常工作时的效率最大是多少？