During and after the financial crisis of 2007minus​2009, the Fed greatly increased the supply of reserves through three rounds of quantitative easing by A. purchases of​ long-term Treasury securities and sales of​ mortgage-backed securities. B. sales of both​ long-term Treasury securities and​ mortgage-backed securities. C. sales of​ long-term Treasury securities and purchases of​ mortgage-backed securities. D. purchases of both​ long-term Treasury securities and​ mortgage-backed securities.