The accompanying figure shows the effects of different treatments of drinking water on the incidence of typhoid fever in Philadelphia, 1890-1935. If filtration of drinking water caused such a dramatic decrease in the disease incidence, was it necessary to introduce chlorination a few years later? Why or why not?

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700 600 500 400 8 300 200 E 100 0 Filtration of drinking water Capture ImageP Chlorination of drinking water started 1890 1900 1910 1920 1930 Year