The Synonymous Substitution Rates Across a Given Genome Rarely Differ

The synonymous substitution rates across a given genome rarely differ by more than a factor of two, a difference that cannot account for the roughly 1,000-fold difference in nonsynonymous substitution rates observed between some genes. This observation strongly suggests

A) that nonsynonymous substitutions are more common than synonymous substitutions.
B) that natural selection has been acting on such genes.
C) that genetic drift is a stronger evolutionary force than previously believed.
D) that mutations rates are far more variable than previously believed.
E) that synonymous substitution rates are underestimated.