Why can we use the "disappearing x" as a method to know when the reaction is complete?

Embryos of different species look very similar, which shows that the organisms share a ancestor.

Which statement is true about this reaction? 14n+1h 15o it is a practical source of energy on earth. it occurs only outside the solar system. its product is heavier than each of its reactants. it shows the critical mass of an element.

In which reaction is precipitation occurring?