Under what circumstances is mass by difference employed?

Iknow #2 is d. i just need to know why

Achemist 16 drop copper metal from copper chloride solution. the chemist place is 0.50 g of aluminum foil in a solution containing 0.75 g of copper (ii) chloride. a single replacement reaction takes place. which statement explains the maximum amount of copper that the chemist can extract using this reaction?

What term is applied to a scientist who studies ancient life, including animal and plant fossils a. anthropologist b. dendroclimatologist c. geophysicist d. paleontologist