Diversable risks

In broad terms, why is some risk diversifiable? Why are some risks non diversifiable? Does it follow that an investor can control the level of unsystematic risk in a portfolio, but not the level of systematic risk?2. Suppose the government announces that, based on a just-completed survey, the growth rate in the economy is likely to be 2 percent in the coming year, as compared to 5 percent for the past year. Will security prices increase, decrease, or stay the same following this announcement? Does it make any difference whether the 2 percent figure was anticipated by the market? Explain.