单选题：Their cheery song brightens many a winter's day. But robins

Their cheery song brightens many a winter's day. But robins are in danger of wearing themselves out by singing too much. Robins are singing all night一as well as during the day,British-based researchers say.
David Dominoni, of Glasgow University, said that light from street lamps, takeaway signs and homes is affecting the birds' biological clock, leading to them being wide awake when they should be asleep.
Dr Dominoni, who is putting cameras inside nesting boxes to track sleeping patterns, said lack of sleep could put the birds'health at risk.His study shows that when robins are exposed to light at night in the lab, it leads to some genes being active at the wrong time of day. And the more birds are exposed to light, the more active they are at night.
He told people at a conference, "There have been a couple of studies suggesting they are increasing their song output at night and during the day they are still singing. Singing is a costly behaviour and it takes energy.So by increasing their song output, there mtgrit be some costs of energy."
And it is not just robins that are being kept awake by artificial light. Blackoiids and seagulls are also being more nocturnal. Dr Dominoni said, "In Glasgow where I live, gulls are a serious problem.I have people coming to me saying `You are the bird expert. Can you help us kill these gulls?'.During the breeding(繁殖)season,between April and June,they are very active at night and very noisy and people can't sleep."
Although Dr Dominoni has only studied light pollution,other research concluded that robins living in noisy cities have started to sing at night to make themselves heard over loud noise.
However, some birds thrive(兴旺)in noisy environments. A study from California Polytechnic University found more hummingbirds in areas with heavy industrial machinery. It is thought that they are capitalising on their predators(天敌)fleeing to quieter areas.
According to Dr Dominoni's study,what causerrobins to sing so much? A．The breeding season.
B．The light in modem
C．The dangerous environment.
D．The noise from heavy machinery.