Which of the following hypotheses is consistent with the observed correlation between tRNA abundance and codon usage in highly expressed genes?

a) Mutations at synonymous sites can experience selection for translational efficiency.
b) Selection on functional genes is weaker than selection on pseudogenes.
c) Rates of substitution at synonymous sites are lower than substitution rates at nonsynonymous sites.
d) tRNA abundance is evolving neutrally.
e) This correlation only tells us which codons are used more frequently; it has no implications for selection.