In the USA children start school when they are five years old． In some states they must stay in school (11) they are sixteen． Most students are seventeen or eighteen years old when they (12) school． There are two kinds of schools in the United States: public schools and pri-cate(私立)schools．(13)children go to public schools． Their parents do not have to(14)

their education because the schools (15) money from the government． If a child goes to a private school，his parents have to get enough money for his schooling ． Some parents still (16) private schools，though they are much more expensive．

Today about half of the high school students(17) unwersities after they finish the secondary school． A student at a state university does not have to pay very much if his parents (18 in that state． But many students(19) while they are studying at universities．In this way they (20)good working habits and live by their own hands．( )11．

A．and

B．though

C．until

D．since