Dna from any source contains about the same amount of guanine

Cytosine

Explanation:

DNA come in base pairs. According to the Chargaff's rule on base pairs, DNA base pairs come in a 1:1 ratio. The base pair of guanine is cytosine (Adenine pairs with Thymine). The 1:1 ratio indicates that for every 1 guanine there is 1 cytosine. So this means that there is an equal amount of guanine and cytosine.

c

explanation:

idk

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.