Analysis of test questions: A, only a balanced diet can reasonably intake all kinds of nutrients needed to ensure human health, which is correct. B, kelp is rich in iodine, which can supplement iodine for human body. This item is correct. C, preservatives added too much, will affect human health, this error. D, sodium bicarbonate can generate carbon dioxide when heated, and carbon dioxide expands when heated, making food soft. This item is correct. So you should choose item C.