听力原文： In the United States, (29) boys and girls start school when they are five years old. In some stales they most stay in school until they are sixteen. Most students are seventeen or eighteen years old when they graduate from secondary school.
Most children go to public elementary and secondary schools. The parents of public school pupils do not have to pay directly for their children's education because (30)tax money supports the public schools. If a child attends a private school, his parents pay the school for the child’s education.
Today about half of the high school graduates go on to colleges and universities. Some colleges and universities receive tax money from the government. A student at a state university does not have to pay very much if his parents live in that state. Private colleges and universities are expensive, however. (31) Almost half of the college students in the United States work while they are studying. When a student's family is not rich, he has to earn money for part. of his college expenses.
(30)

At 5 years old.
B. At 6 years old.
C. At 7 years old.
D. It's not clear.