The graph shows how the average surface water temperature f of Lake Michigan varies over the course of a year (where t is measured in months with t = 0 corresponding to January 1). The average was calculated from data obtained over a 20-year period ending in 2011. Sketch the graph of the derivative function f'. When is f'(t) largest?

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f4 (°F) 70+ 60+ 50 40- 6 8 10 12 t (months) +00 4. 2.