American schools begin in September after a long summer holiday. There're two terms in a school year. The first term is from September to January, and the second term is from February to June. Most American children begin to go to school when they're five years old. Most students are seventeen or eighteen years old when they finish high school.
      High school students take only five or six subjects each term. They usually go to the same classes every day, and they have homework for every class. After class, they do a lot of interesting things.
     After high school, many students go to college(大学). They usually have to pay a lot of money. So, many college students work after class to get money for their studies.

世界各地凡华人居住的地方多被称为“唐人街”，中国人常被称为“唐人”。这名称的由来是因为（    ）

二项式  ${\left(x-\frac{1}{x}\right)}^9$的展开式中 x³ 的系数是（    ）

伞藻是单细胞藻类，由帽、柄、足3部分组成，细胞核位于足中。下图表示伞藻的嫁接实验。该实验结果表明