Why is Radio 3 used for the spectrum analysis?

Refer to the exhibit.

Why is Radio 3 used for the spectrum analysis?
A . The 5 GHz frequency band is available only on Radio 3.
B . Radio 3 is the configured dedicated monitoring radio for this FortiAP model.
C . Only Radio 3 is compatible with the selected band.
D . Radio 1 and Radio 2 are unavailable to run the spectrum analysis.

Answer: B