Rifampicin is known to inhibit RNA synthesis in E.coli.Once the first ten nucleotides have been incorporated,rifampicin does not inhibit RNA synthesis.Which of the following best explains how rifampicin functions?

A)rifampicin binds only to the core enzyme when bound to DNA but can bind to the holoenzyme when not bound to DNA
B)rifampicin can bind to RNA polymerase when the polymerase is not bound to DNA but cannot bind when the polymerase is bound to DNA
C)rifampicin only inhibits when the $\alpha$ subunit is bound to the core enzyme
D)rifampicin is able to bind to the nucleotide binding site at any time
E)none of the above