Is about 3 hours to my understanding
Answer:
Oil
Explanation:
Since, unlike oil, source like wind, solar, water etc. come naturally we don't and can't replace them but when the viscosity in oil has been used up it must be change to allow the proper functioning of the machine.
Different enzymes have specific active sites which can only catalyze one <span>substrate. Enzyme-substrate complexes are produced only when the substrate fits into the active site :)</span>
Animal dander would be the answer you're looking for.
Hope this helps :)
