(1) charcoal powder and oxygen combustion to generate carbon dioxide gas, will affect the results of the experiment, so guess a must be wrong; fill in: charcoal powder and oxygen combustion to generate carbon dioxide gas, will affect the results of the experiment;
(2) iron powder can be further and the beaker in the low content of oxygen reaction, so the addition of iron powder to make the results more accurate. Therefore: iron powder can further react with oxygen in the beaker.