Economists use changes in gdp to measure

Economists use changes in GDP to measure the state of a country's economy.  The gross domestic product, also known as GDP, is a tool that economists around the world use to measure how the economy of a specific country is doing. They use this tool, because it represents the value in american dollars, of all the services and goods that a country produced during a specific amount of time. This number gives an estimation on how big or small the country's economy is.

the answer is b your welcome : )

new evidence is discovered.

oversights and errors are corrected,

new technology is applied to evidence.