Answer
a. a micromineral or trace elements is a mineral needed in small amounts, less than 100mg, they are almost all transition elements in period 4( $Fe$,$Zn$,$Cu$,$Cr$,$Co$,$V$) and $Si$,$Mo$,$I$,$Se$,$As$
b. Iodine is important for thyroid function
c. 20mg
Work Step by Step
a. you can find this answer on the second paragraph of the topic "Chemistry Link to Health: Elements Essential to Health", page 108
b. you can find this answer at page 109, table 4.4
c.you can find this answer at page 109, table 4.4
