18. an atomic mass unit (amu) has a mass equal to about a)1/2,000 that of a neutron b)that of a proton c)that of an electron d)2,000 times that of a neutron

b

Explanation:

the reaction mixture contains some ammonia, plus a lot of unreacted nitrogen and hydrogen. the mixture is cooled and compressed, causing the ammonia gas to condense into a liquid. the liquefied ammonia is separated and removed. the unreacted nitrogen and hydrogen are then recycled back into the reactor .

c is the answer because when something is left after something evaporates that shows the chemical change.