Just a bit more background on (b) - from the paper in Phys. Rev. Lett. (https://dcc.ligo.org/LIGO-P150914/public https://dcc.ligo.org/LIGO-P150914/public): "On

Just a bit more background on (b) - from the paper in Phys. Rev. Lett. (https://dcc.ligo.org/LIGO-P150914/public https://dcc.ligo.org/LIGO-P150914/public): "Only the LIGO detectors were observing at the time of GW150914. The Virgo detector was being upgraded, and GEO 600, though not sufficiently sensitive to detect this event, was operating but not in observational mode." The GW150914 event was the strongest one observed in the period LIGO was operating: "Detected with ηc = 20.0, GW150914 is the strongest event of the entire search." So, the strongest event happened to occur when the other detectors were not running.