On Earth, how can you tell up from down?

Gravity

Explanation:

On earth there is always Gravity, so if its pushing you down you have your answer.

Well if your talking ABOUT the earth then they use satellite imagery and scan the earth, and I’m pretty sure they would know the bottom from the top because of the contents and such. But when i think about it there really isn’t a up and down on the earth cause it’s a sphere, or maybe the moon is the bottom and the sun is the top.

I know the answer is
A: 2.5 m/s2
Just took the test.