题目

根据短文理解，选择正确答案。 Idioms(成语) are phrases and sentences that do not mean exactly what they say. Even if you know the meaning of each word you see or hear, you may not understand the idiom because you don't understand the culture behind it. For example, if an American boy asks his mother what's for dinner tomorrow, she may say “I'll play it by ear”, that means she doesn't have plans for dinner and she will decide later. “Play by ear” used to mean playing music using the sheet music(活页乐谱), but now people often use it when they're not talking about musi1700年美国的黑种人口只有2.7万人，1820年达到177.2万人。黑种人在美国剧增的主要原因是 A．新航路开辟                                             B．自然增长 C．战争中的难民流亡                                   D．三角贸易