青藏高原夏季气温比长江中下游低得多，其主要原因是

阅读下列短文, 从每题所给的四个选项（A、B、C和D)中, 选出最佳选项。

A

    What is a Chinatown? Is it a town in China? Of course not! It is in a foreign(外国的) country. It is a place where many overseas(海外的) Chinese live together.

    There are lots of Chinatowns in America. The biggest one is in New York. There are about 150,000 people in it. It is the home of Chinese New Yorkers. And it is also a good place to go.

    Shopping in Chinatown

    Shopping in Chinatown is fun. You can find lots of things there. Most of them are made in China. Mott Street is the best place to shop because the things there are very cheap.

    Eating in Chinatown

    Chinatown has more than 200 restaurants. You can enjoy traditional (传统的) Chinese food there. New Silver Palace Restaurant is one of the best restaurants. Why don't you have breakfast with a cup of green tea in it? You can eat some Shanghai food in Evergreen Restaurant.

    Living in Chinatown

    Living in Chinatown is comfortable. You can find hospitals, schools, banks, libraries and supermarkets in it. In some schools, students must study Chinese. Many overseas Chinese send their children to these schools. They want their children to learn more about China and its culture(文化).

若对于实数a，b，规定a*b=$\left\{\begin{array}{c}{a}^2-ab\left(a\ge b\right)\\ ab-a^2\left(a<b\right)\end{array}\right.$，例如：2*3，因2＜3，所以2*3=2×3﹣2²=2．若x₁，x₂是方程x²﹣2x﹣3=0的两根，则x₁*x₂=____ .