Answer: A
Explanation: Beta particles have a charge of -1
Electron - negligible mass, negative charge, orbits the nucleus
Proton - 1 AMU, positive charge, in the nucleus
Neutron, 1 AMU, no charge, in the nucleus
As the gas cools it condenses and becomes a liquid its atoms also become smaller
I'm pretty sure it's the same 2mm = 2cc = 2c^2