<Tr> <Td> Oxygen </Td> <Td> O </Td> <Td> 209,460 </Td> <Td> 20.946 </Td> </Tr> <Tr> <Td> Argon </Td> <Td> Ar </Td> <Td> 9,340 </Td> <Td> 0.9340 </Td> </Tr> <Tr> <Td> Carbon dioxide </Td> <Td> CO </Td> <Td> 400 </Td> <Td> 0.04 </Td> </Tr> <Tr> <Td> Neon </Td> <Td> Ne </Td> <Td> 18.18 </Td> <Td> 0.001818 </Td> </Tr>

Which of the following is not a major section of the atmosphere