According to google. It is a substance produced by a living organism that acts as a catalyst to bring about a specific biochemical reaction.
D is the answer (Aa and aa ) i hope this helps
Go through this link it is has all your answers. What it is and examples. Hope this helps! (: http://study.com/academy/lesson/elastic-potential-energy-definition-formula-examples.html
I am fairly sure the answer is B
