Studies Documenting the Arrival of Spring and Fall in the Northern

Studies documenting the arrival of spring and fall in the Northern Hemisphere are based on:

A) average daily temperatures as measured at locations throughout Northern Hemisphere.
B) fixed calendar dates that have been consistent for the past 150 years.
C) when buds of particular plants open and when the leaves of particular plants change color and fall.
D) average snow melt, precipitation, and consistently increasing or decreasing temperatures across the state.
E) the amount of pollution, particularly greenhouse gases, found in the atmosphere of that region.