The RNA Polymerase Holoenzyme Consists of Which of the Following $\alpha$

The RNA polymerase holoenzyme consists of which of the following subunits?

A) ( $\alpha$ 2, $\beta$ , $\beta$ ' )
B) ( $\alpha$ 2, $\beta$ , $\beta$ ' $\sigma$ )
C) ( $\alpha$ , $\alpha$ ', $\beta$ , $\beta$ ' )
D) ( $\alpha$ 2, $\beta$ , $\beta$ ', $\rho$ )