Will an enzyme work on any substance?

What causes darkening of the skin as melanin production increases

Stephen is a student who wants to test his knowledge of medical terminology. which option could he use?

Ihave no idea how to solve these. plus there's another question that is 2) a 10 kilogram meteorite is collected in space and brought back to earth. it is composed of 1.25 kg of uranium-238 and 8.75 kg of lead-206. how many u-238 half lives have passed? what is the approximate age of the meteorite?