气瓶的压力主要来源于（）。
参考答案：瓶装气态分子不停地对瓶壁的碰撞
— Do you know a Smith?
— Yes. You can't imagine man living next to me is . [ ]
A. a; a poet and novelist
B. a; a poet and a novelist
C. the; poet and novelist
D. the; a poet and novelist
自1840年以来，西方列强侵略中国，中 * * 备受屈辱。下列侵略战争曾经攻占北京城的是（ ）
①鸦片战争 ②第二次鸦片战争 ③甲午中日战争 ④八国联军侵华战争
A．①②
B．③④
C．①③
D．②④
