A、By selling services that make life comfortable. B、By selling land containing oil. C、By selling public lands. D、By selling coal

[18] We use all sorts of services without thinking about how we get them. But such services cost money. We pay for them through taxes. What would happen if everyone in the city stopped paying taxes? The water supply would stop, the street might not get cleaned, and there would be no police force to protect people and property. [16] The chief duty of every government is to protect persons and property. More than three fourths of the money spent by our government is used for this purpose. The next largest amount of public money goes to teach and train our citizens. Billions of dollars each year are spent on schools and libraries. Public money is used to pay the teachers and other public officials. [17] Years ago the government made money from the sale of public lands. But most of the best public lands have now been sold. The money raised was used to help pay the cost of government. There are still some public lands that contain oil, coal, gas and other natural products. They could be sold, but we want to save them for future years. So we all must pay our share for the services that make our lives comfortable.
Questions 16 to 18 are based on the passage you have just heard.
16. What is the chief duty of every government?
17. How did the government raise money in the past?
18. What is the passage mainly about?