单选题 Taxes consist of the money which people pay to support their government. There are generally three levels of government in the United States : federal, state, and city ; therefore, there are three types of taxes.
Salaried people who earn more than a thousand dollars must pay a certain percentage of their salaries to the federal government. The federal government has a graduated income tax; that is, the percentage of the tax increases as a person's income increases. With the high cost of taxes, people are not very happy on April 15, when the federal taxes are due.
Americans pay their federal taxes

• A. on the 15^th of each month.
• B. on the first day of each month.
• C. once a year.
• D. twice a year.