Thermal windows often have all the air pumped out between the panes and a gas pumped in that is not harmful and wont react with othet things. what gas would be used

C. Ar or Argon

It is the most commonly used gas between thermal window panes. Argon has 34% lower thermal conductivity than air and is therefore more suitable to be used instead of air. It is also inert and does not react or causes any form of harm.