Trace elements and health

A, iodine is the main element in the synthesis of thyroid hormone, insufficient or excessive will cause goiter, so the choice is correct.

B, selenium has anti-cancer and anti-cancer effects. Selenium deficiency may cause epidermal keratinization and canceration, and the option is wrong.

C, fluoride can prevent and treat dental caries, lack of susceptible to dental caries, so the choice is wrong.

D, iron is the main element of hemoglobin synthesis, lack of it will lead to anemia, so the option is wrong.

So choose a.