Which of the following should have a hyphen?
a. nineteen
b. written quickly
c. american president
d. self evident

Self-evident (According to Oxford Dictionaries)

Written quickly is two words. Nineteen is a number and the way it is spelled is correct and does not need a hyphen. American president doesn't need a hyphen either.

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.