"The decision by the administration to back away from its missile defense commitment to the Czech Republic and Poland can only demonstrate to the rest of Europe that the United States is not prepared to stand behind its friends, that the United States views resetting its relationship with Russia more important than commitments made to close friends and allies, and that the administration is willing to let Russia have veto power over the disposition of our missile defense architecture,".

VOA: standard.2009.09.24

The second way to have something that is net nonpolar is to have spatially symmetric disposition of polar bonds.

第二种构成,需要空间非极性,就是需要极性键的空间对称分布。

麻省理工公开课 - 固态化学导论课程节选