Living in the city will change you sooner or later. You often get louder, ruder, and use slang that outsiders don't understand. These classic　 16 　of urban living can also be seen in birds. Like people, birds that live in the city are louder,

　Living in the city will change you sooner or later. You often get louder, ruder, and use slang that outsiders don't understand. These classic　 16 　of urban living can also be seen in birds. Like people, birds that live in the city are louder,　 17 　.and more stressed out than their country cousins. 　A study found that city birds start singing earlier than　 18 　in the countryside. Researchers observed that sparrows start singing earlier　 19 　not because of light pollution, but to avoid noise from the traffic.　 20 　every city resident, the birds have learned that you need to start your day early if you want to avoid the rush hour. City-dwelling species like blackbirds　 21 　their volume or alter their songs to　 22 　amid the noise of the bustling metropolis. 　 23 　rural birds may not be able to make sense of 　 24 　their big-city cousins are singing about. 　Rural birds may also find their city cousins rather rude. In crowded places, urban sparrows are more　 25 　because, although there is more food in cities, it is often more work to claim it and keep it away from other　 26 　. All this hustling makes city birds, like its people, smart. In a recent study, bullfinches living in cities　 27 　their country cousins in cognitive tests, by acting more boldly and creatively when presented with unfamiliar or difficult-to-reach　 28 　of food. 　City birds might be louder, tougher, and even smarter. Compared to their rural　 29 　.they also die younger. Researchers found that their cells aged　 30 　than their country cousins. They have to cope with a lot of stress.________

A.symbols

B.sights

C.symptoms

D.signals

正确答案是C