There is no ELEMENT with the symbol "DO" however, it would be the molecule which is one atom of deuterium (D) and one atom of oxygen (O). Deuterium is an isotope of hydrogen. This molecule would be called deuterium oxide.
iron
Rubidium.
Nitrogen
Chlorine.
Hydrogen
there is no such element
lithium
Carbon
Barium
Arsenic
Titanium
No element has the symbol A