题目

Violin prodigies (神童), I learned, have come in distinct waves from distinct regions. Most of the great performers of the late 19th and early 20th centuries were born and brought up in Russia and Eastern Europe. I asked Isaac Stern, one of the world’s greatest violinists the reason for this phenomenon. “It is very clear,” he told me. “They were all Jews and Jews at the time were severely oppressed and ill-treated in that part of the world. They were not allowed into the professional fields, but they were allowed to achieve excellence on a concert stage.” As a result, every Jewish 命题“存在一个三角形，内角和不等于1800”的否定为A.存在一个三角形，内角和等于1800B.所有三角形，内角和都等于1800C.所有三角形，内角和都不等于1800 D.很多三角形，内角和不等于1800