共建和谐世界，必须坚持包容精神。文明多样性是人类社会的基本特征，也是人类文明进步的重要动力。各种文明有历史长短之分，但没有高低优劣之别。在文化上各国应相互借鉴、共同繁荣，而不应排斥其他民族文化。这是 [ ]

A．实施不结盟战略的要求

B．缓和国际紧张局势

C．建立国际新秩序

D．和平共处五项原则

When I try to understand () that prevents so many Americans from being as happy as one might expect, it seems to me that there are two causes.

A. why it does

B. what it does

C. what it is

D. why it is