The number of bachelor's degrees conferred in the United States has been increasing steadily in recent decades. Based on data from the National Center for Education Statistics, the rate of change of the number of bachelor's degrees (in thousands) can be approximated by the functionB'(t) = 0.0612t^2 -1.588t +16.14a. Find B(t), given that about 841,700 degrees were conferred in 1970 (t=0).