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.

the median of the data is 4.

step-by-step explanation:

the median means the middle number of a given number.

to decide which one is the median, arrange in ascending or descending order. but sometimes, you may find two(2) number in the middle, but it's easy, just add them together and divide by 2.

so,   determine the median of the data. 2, 3, 3, 4, 5, 7, 9

it is already arranged, count 3 times forward and 3 backward and the one left is your answer.

2,3,3,4,5,7,9

our answer is 4.

hope this !