In the United States , most children begin attending school when they are five years old . Public education is free and most children go to schools near their homes . Schools in these early years is fun . The children learn to read and write . But they also play games and go on trips . There is no reason at such an early age for him to hit the books.     The situation changes as children become older. The subjects become more difficult .Students must learn about world history , algebra   , and the life sciences. Tests become more common . Pressure is growing to hit the books, study hard , and advance. However , most serious students only really begin hitting the books for long hours when they reach high school.     High school students who hope to continue their education at a college or university must take a special national test called the S-A-T .The test has two parts .The first part tests the student's ability with numbers and mathematical skills .The second part tests the student's ability in the English language.     The S-A-T test is very important .A high school student who gets a high score on the two parts of the test has a good chance to enter a top American college .For this reason, many students hit the books for months to prepare for the S-A-T. Form this passage ,we can clearly know "hitting the books" means  _
working hard for the future studying a subject carefully reading books in a hurry reading books with great effort