U.S. federal tax revenue is the total tax receipts received by the federal government each year. Most of it is paid either through income taxes or payroll taxes. In fiscal year (FY) 2021, income taxes will account for 50%, payroll taxes make up 36%, and corporate taxes supply 7%. The rest is made up of estate taxes, excise and custom duties, and interest on the Federal Reserve's holdings of U.S. Treasurys.
I'd say the 1st one: Carbon decays at a constant rate, and scientists measure the amount of carbon in a fossil to determine its age. (Half life of 5730 years)
