For a riboswitch that controls transcription, what is the effect of a regulatory molecule such as TPP?

a. It binds to a repressor and causes the repressor to bind to an operator site.
b. It binds to an activator and causes the activator to bind to an operator site.
c. It binds to an RNA molecule and causes a change in its conformation.
d. It binds directly to the promoter and prevents transcription.