Which equation shows a conservation of mass?
(1) na + cl2 --> nacl
(2) al + br2 --> albr3
(3) h2o --> h + o2
(4) pcl5 --> pcl3 + cl2

with this one i know the answer in craters one reason i looked it up and i learned about it there is my evidence

metals are good conductor of electricity. it has free electrons on their surface. according to electron pool theory, each atom present in a metal crystal losses all of its valence electron and as a result a pool of free is form. these pool of electrons are free to move and   conduct electricity easily   while making metal as good conductor.

the correct answer is a. the ability of electrons to flow throughout the metal.

if it's not useful, feel free to report for mistake in answer.