How is the highly reactive nature of chlorine a benefit in the treatment of sewage?

Which statement is best described as a atom

Can someone me with 1 to 4 plz. fast

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.