油品中烃类氧化后生成的中性物质会与灰尘、机械杂质混在一起形成沉淀物,对变压器产生有害的影响()。
参考答案:对
His sister had to wait for him for another two hours, _______ she? [ ]
A. wasn't
B. doesn't
C. hadn't
D. didn't
①用刻度尺多次测量某一物体的长度;
②在做“探究凸透镜成像”实验时,多次改变物距观察成像情况;
③研究杠杆平衡条件时多次调节钩码的个数及位置;
④在做“伏安法测电阻”实验时,调节滑动变阻器,测量对应的多组数据.( )
A.①②
B.②③
C.③④
D.①④