The CAPM can be used to find a business's expected opportunity cost of capital: ri=rf+i(rmrf) What should be used as the risk free rate rf? (a) The current central bank policy rate (RBA overnight money market rate). (b) The current 30 day federal government treasury bill rate. (c) The average historical 30 day federal government treasury bill rate over the last 20 years. (d) The current 30 year federal government treasury bond rate. (e) The average historical 30 year federal government treasury bond rate over the last 20 years.