Compare a sled's potential energy at 10 m to its potential energy at 20 m. how does doubling height affect potential energy?

It effects it exponentially. Whereas you can get to a point where an object is so high that when dropped it will achieve terminal velocity. Without the factors of drag and just all around friction in the equation, all objects will fall at a rate of about 9.8 meters per second. But they must accelerate to that speed. Doubling the high adds more time for the sled to achieve its maximum speed. Meaning it's potential energy increases. 

Julia would be observing a. static electricity was caused by her sweater rubbing against her shirt and transferring electrons. she didn't touch anything metal, so that would cancel out b.she most likely didn't rub her sweater against her hair either, so that cancels out c.she also didn't scoot across the carpet while trying to take her sweater off, that cancels out d.so that leaves a.a. is correct.