The mass number of an atom is obtained by totaling the number of and electrons and protonselectrons /neutronsprotons and neutrons/ neutrons and isotopes

i'm assuming that the correct answer is indeed the first one/ "a"..

in 1783 cavendish was working with his hydrogen gas and found that when he burned it and condensed the vapor it was water. this was very significant since it disproved the greek theory of elements completely since water was not a simple substance but the product of two gases. it was lavoisier who hearing of the experiment gave cavendish's gas the name hydrogen ("water-producer") and added that hydrogen burned by combining with oxygen and that water was a hydrogen-oxygen combination.

explanation: