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

B. but the relative amounts of G+C and A+T vary by species.

Explanation: Chargaff's rule states a relantionship between guanine (G) and cytosinev(C) amounts and adenine (A) and thymine amounts (T). But these amounts could  change  across species, so much so, GC content is a characteristic of each species genome. For example, there some bacteria like Actinobacteria classified as " high GC bacteria", a plant Arabidopsis thaliana has 36% of GC content and others like Plasmodium sp. are classified as "high AT content".

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