Possible answers:
A). PV=nRT
B). heat or cold
C). reactants
D). energy barrier
E). Dehydration
F). harmful substances
G). resistance
H). temperature

Fact 1: All chemical reactions require activation energy to get started. Fact 2: Activation energy is required to overcome the energy barrier. Given the two facts above, what must a chemical reaction overcome before starting?
Answer:
D).