热机是将燃料燃烧获得的内能转化为____ 能的装置；火箭发射用“液氢”作燃料，是由于氢的____ 大。

$2+\sqrt{3}$和$2-\sqrt{3}$的等比中项是   （    ）

    In the United States, children start school when they are five years old. In some states they must stay in school1they are sixteen. Most students are seventeen or eighteen years old when they2secondary(中等的)schools. There are two kinds of schools in the United States: public(公立的)schools and private(私立的)schools.3children go to public schools. Their parents do not have to4their education because the schools5money from the government(政府).If a child goes to a private school, his parents have to get enough money for his schooling. Some parents still6private schools, though they are much more expensive.

    Today about half of the high school students7universities(大学)after they finish the secondary school. A student at a state university does not have to pay very much if his parents8in that state.

    But many students9while they are studying at universities. In this way they10good working habits(习惯)and live by their own hands.