Studies documenting the arrival of spring and fall in the northern hemisphere are based on

Spring - When buds of particular plants open

Fall - When the leaves of particular change color and fall.

Explanation:

The arrival of the spring and fall in the Northern hemisphere is base on nature, especially through trees. During fall, trees start to change its leaves colors become visible and give the leaves a part of their fall splendour. The breaking of chlorophyll causes leaves to turn into orange, yellow, and in some places in reddish and purplish. The arrival of the spring happens when fresh buds bloom, and the earth seems to come to life again with grasses and plants. Spring is the season of new beginnings.

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1) d

2)c

3)b

4) a

5)c

6) c