What causes the aurora borealis or northern lights to occur?

-streams of charged particles from the sun mix with earth's atmosphere
-light from the moon reflects on earth's atmosphere
-charged particles from the sun reflect off of the ice at the poles
-none of the above

The correct answer to your question is A. : Streams of charged particles from the sun mix with Earth's atmosphere.  I took a test with this same question, i got it correct!

The primarily goals of the zapatistas of mexico in 1994 were land reform and redistribution. it is also included in their demands to have a greater political and cultural autonomy for the indigenous people of chiapas and the rest of mexico.

This is all you, but i can give you some topics that you could try. if it rained when you want to do something outdoors, it 'limits' that human activity. maybe you want to do an experiment that goes throughout the whole year and you have to have above freezing the entire year. this 'limits' your activity. i thought of those just off of the top of my head. something that gets to me that might be getting to you is you are thinking too hard about what to do. think simply, and if it is too simple, then bump it up just a little bit.