What is the role of mediator with regard to eukaryotic gene regulation?

a. Mediator controls the switch to the elongation phase of transcription.
b. Mediator controls whether or not RNA polymerase can bind to the core propoter.
c. Mediator causes histones to be displaced from the core promoter.
d. Mediator controls whether or nol TFIID can bind to the core promoter.