Which one of the following is true about gene regulation?

a. Methylation of cytosine occurs on one strand in CpG dinucleotides.

b. The mediator protein binds into the DNA major groove located on the sides of the base pairs.

c. Activator proteins bind the TATA box sequence to upregulate gene expression.

d. In eukaryotes, basal gene transcription is low and the majority of gene regulation is positive.