The gross national income (GNI) of a certain country (in billions of U.S. dollars) can be approximated by f(t) = 1020 e 0.154t `, where t= 0 corresponds to the year 2000 (a) Find f' (t). (b) At what rate was the GNI changing in 2000? (c) Repeat part (b) for 2013. (a) f' (t) = (b) In 2000, the GNI was changing at a rate of about $ billion per year. (Round to the nearest integer as needed.) (c) In 2013, the GNI was changing at a rate of about $ billion per year. (Round to the nearest integer as needed.) O