"In practice, the lags . . . make monetary and fiscal policy difficult to apply. The lags and the general complexity of the national economy also make it difficult to sort out how different policies have affected the economy in the past. Economists have debated these matters for many years." This statement explains which of the following?
a. why most economists today are monetaristsb. why the national debt has grown so alarminglyc. why the business cycle existsd. why there is often disagreement over monetary and other economic policies