在机车底部作业前，应确认机车（），并在机车（）.
参考答案：防滑；前、后放臵铁鞋
对于铁钉、玻璃、水银、食用油和陶瓷片这些物品，在常温下是液体又是晶体的是______，是导体又能被磁铁吸引的是______．
25. Only when I lost the opportunity to take my first job ____ how important it was to me.
A. I realized B. I had realized C, had I realized D . did I realize
I realized D . did I realize