Answer:
Your answer is B, Electrochemistry!
Explanation:
This is the part of chemistry that studies the chemical process in which electrons flow. This flow is called electricity. Electricity is generated by the flow of electrons, from one element to another element. This reaction is called oxidation reduction.
Well that is because the stratosphere has lower temperatures and low air density. This gives the airplane less air pressure and with low air density they can fly faster :) Hope this helps.