These are Chargaff's rules that is why cytosine bind to guanine, while thymine bonds to adenine. Thus, the correct option is A.
According to the rule, in DNA there is always equality in quantity between the bases A and T and between the bases G and C. (A is adenine, T is thymine, G is guanine, and C is cytosine).
According to Chargaff's rule, Concentration of adenine = concentration of thymine. therefore if [A]=40% then the [T] will also be 40%.
For more information regarding Chargaff rule, visit:
https://brainly.com/question/14838724
#SPJ1