near the magnetic poles of the the Sky glows with The Spectacular display of green and pink lights why does this happen

Iwill give brainliest if you answer right : )

At very high pressures, gases become and will eventually a) more dense; become hotter b) more dense; change to a liquid or solid c) less dense; combust d) less dense; turn into a liquid

Which of the following is the best name for mgcl2