质心
参考答案:
以和牛顿第一定律一致的等速度移动的孤立系统中的一点。
People in English-speaking countries usually greet each other in this way "______". [ ]
A. Have you eaten?
B. A nice day, isn't it?
C. B and D.
D. Nice to see you.
爱是源于生理上的激情、持续于心灵上的伙伴和坚定的承诺,三者缺一不可。