Chargaff's rule states that the amounts of guanine and cytosine are roughly the same, and the amounts of adenine and thymine are roughly the same

a. in eukaryotes only. the rule doesn't apply to prokaryotes.
b. but the relative amounts of g+c and a+t vary by species.
c. except in species that use uracil in place of thymine in their dna.
d. and the relative amounts of g+c and a+t don't vary by species.