Afourminusfirm concentration ratio measures a. the fraction of an industry's sales accounted for by the four largest firms. b. the fraction of employment of the four largest firms in an industry. c. how the four largest firms became so concentrated. d. the production of any four firms in an industry.

A four-firm concentration ratio measures the fraction of an industry's sales accounted for by the four largest firms. A four-firm concentration ratio compares different companies within the same market and the type of control they have over it. The control is in relation and indication of an oligopoly that the companies create together. 

The federal reserve took an expansionary approach during the crisis. this was done by expanding the money supply and boosting liquidity. this can be seen in the fed's actions of lending to banks, purchasing securities, and lowering the federal funds rate in order to lower overall interest rates. the fed's goal was to increase consumer spending and overall liquidity within the system, and they pursued this by expanding the supply of liquid money.