氮气是一种白色、无味、有毒的气体。
参考答案：错
解析：氮气是一种无色、无味、无毒的气体。
为防止交叉感染，哪位病人应首先换药()。
A．脓肿切开引流
B．下肢慢性溃疡
C．冻疮溃烂
D．清创缝合后拆线
E．褥疮
After people have learned that magnets attract things, centuries passed () they took note of the fact that magnets sometimes also repel things.
A. before
B. until
C. after
D. since
