"Plants could readily develop seasonal responses for flowering based on temperature changes but have "elected" through evolution to manage their rhythms based on a comparison of daylight and darkness. Why has the day-night ratio system been selected?"

Temperature is a gradation but day/night is all-or-nothing

Explanation:

Temperature change is progressing. I.e from Hot to cold, it moves gradually along a gradient. But Night and Day is a two edged variance. It's either day or night, also know as the All-or-nothing .

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sulfur dioxide

explanation: acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air. these substances can rise very high into the atmosphere, where they mix and react with water, oxygen, and other chemicals to form more acidic pollutants, known as acid rain.