The graph below shows the sunspot number observed between 1750 and 2000. The graph shows sunspot number on the y axis and years 1750, 1800, 1850, 1900, 1950, and 2000 on the x axis. The graph rises and falls in a cyclic order in gaps of approximately 10 years. The number of sunspots observed in 1750 was 90 and the number of sunspots in 1800 was 10. The number of sunspots observed in 1850 was 130 and the number of sunspots in 1900 was 15. The number of sunspots observed in 1950 was 150. Based on the graph, which of these periods most likely witnessed the greatest fall in average global temperatures?