美国和欧洲也扮演了关键角色。

The role of America and Europe is also crucial.

youdao

要确保具备所有的关键角色。

Make sure that all key roles are filled.

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2011年，政客们将再次扮演关键角色。

In 2011 politicians will again play a pivotal role.

youdao

If these bonds were all completely of equal distance apart, whether is was a lone pair or bonding electrons, 5° the angles would be 109 . 5 degrees.

如果不管它是孤对，还是成键，它们等距分开的话，键角是109。

麻省理工公开课 - 化学原理课程节选

But because there's this lone pair here, it's pushing down on the other bonds, 5° so we end up with an angle of less than 109 . 5 degrees.

但因为这里有孤对，它会把其它键向下压，所以键角会小于109。

麻省理工公开课 - 化学原理课程节选

They want to get as far away from each other possible, the ideal angle is 120. But what we have here is a four-membered ring, so what angle does 90° that have to be, that bond? 90 degrees.

它们想要尽量远离彼此，最理想的是形成120°键角,但现在是个四元环,所以这键角应该多大？

麻省理工公开课 - 化学原理课程节选