It is a rule among people who write for publication never to proofread their own work. why?
they will be tempted to rewrite the text and miss their publication deadline.
they are too involved in the meaning of the text and may not see errors.
most writers are not detail-oriented and will not do a thorough job.

A. child is the answer

Okay, biology isn't exactly my strongest subject (it never has been), but i'm pretty sure the answer is nucleotides. dna is made up of nucleotides. each nucleotide contains a phosphate group, a sugar group, and a nitrogen base. there are four different nitrogen bases: adenine (a), thymine (t), guanine (g), and cytosine (c). a always pairs with t, and g always pairs with c.