Which of these is the equation used to derive a 64 Kbps bit rate?

Which of these is the equation used to derive a 64 Kbps bit rate?

A.
2 x 8 kHz x 4-bit code words

B.
8 kHz x 8-bit code words

C.
2 x 4-bit code words x 8 kHz

D.
2 x 4 kHz x 8-bit code words