Explain the three criteria that are used to determine whether a particular variable is a worthy candidate to be selected as an intermediate target variable of monetary policy. Now, on the basis of these criteria, evaluate the following as potential intermediate target variables:
a) The 90-day Treasury bill yield
b) The real federal funds rate
c) The real 30-year U.S. government bond yield
d) The non-borrowed monetary base
e) The growth rate of M2
f) Excess reserves