The atmosphere is at higher elevations, making the average climate there. a. thicker; cooler b. thinner; cooler c. thicker; warmer d. thinner; warmer

Hey there, you answer would be:

The atmosphere is thinner at higher elevations, making the average climate cooler.  (B)

As you climb a mountain it gets a little harder to breath. This is because the air gets thinner. Meaning its harder to breath in more air.

That's why they need masks when they climb mountains as high up as Mount Everest for example.

It gets cooler as well. It gets a little chilly on some mountains but on higher ones. It snows at the peak. 

Hope this helps!
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Maybe you should get some from your teacher and learn more so you can get your grades up and maybe study a bit more