3. Which one of the following is true? a. The US tax system is voluntary because taxpayers can choose or not choose to file their federal income tax returns without violating the federal law. b. The US tax system is voluntary because taxpayers determine the correct amount of their income and tax rather than the US government determining their income and tax for them. c. None of the above are true . Which one of the following is true? a. When the US Constitution was written, Congress could not directly tax an individual's income unless a census had been taken. b. When the US Constitution was written, Congress could directly tax an individual's income without taking a census if the tax revenues were intended to fund a war. c. All of the above are true