根据短文内容和所给单词提示，写出空白处各单词的正确形式。（每空一词）。
Every society has ____ (it) special words, expressions and ways of saying things. One of the popular expressions in the United States is "Hitting the books". It ____ (mean) studying a subject with great effort(努力).
One of the great challenges for students ____ (be) taking tests. Students spend long hours ____ (prepare) for a test. They read all their books carefully, and then read them ____ second time and a third time. They prepare for every possible question. They study every night. They are "hitting the books."
In the United States, most children go to school ____ they are five years old. Public education is free and most children go to schools near their homes. School in these early years is fun. Children learn ____ (read) and write, but they also play games and go on trips. There is no reason at such an early age for them to hit the books.
This situation changes as children become older. The subjects become ____ (difficult). Students must learn ____ world history, algebra(代数) and life sciences. Tests become more common.
Pressure is growing to hit the books, study hard, and advance(升级). ____, most students only really begin hitting the books for long hours when they reach high school.
Knowledge is a prize that is not easy to gain.
，氟原子在化学反应中易