Each of the Following Is Something That Economists Don't Know

Each of the following is something that economists don't know but about which we are striving to learn except

A) the long-run relationship between kinds of investment and productivity growth.
B) the short-run determinants of investment.
C) why international financial markets have been so vulnerable to financial crises over the past decade.
D) that a high rate of investment is necessary for achieving successful economic growth.