What is a key difference between SRDF/Metro and SRDF/S?

What is a key difference between SRDF/Metro and SRDF/S?
A . SRDF/Metro: R1 devise is host accessible, R2 is red-only
SRDF/S: R1 is host accessible, R2 is write disabled
B . SRDF/Metro: R1 and R2 devices are host accessible at the same time
SRDF/S: R1 devices are host accessible, R2 devices are write disabled
C . SRDF/Metro: R2 can be larger than R1
SRDF/S R2 cannot be larger than R1
D . SRDF/Metro: Requires a third array for any witness functionality
SRDF/S: Does not require a third array

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Answer: B