A new study says that birds living in big cities sing shorter, faster songs that are higher-pitched   than those sung by their brothers in the forests. The researchers think that the birds adjust their songs to allow themselves to be heard over the noises of the city, especially the low traffic noise. To study how urban birds song, Hans Slabbekoorn and Ardie den Bore, biologists at the University of Leiden in Netherlands, travelled around Europe and recorded bird songs in ten major cities and in nearby forests. The species they focused on, the great tit  , is widespread across Eurasia and one of the few types of birds that  _ in big cities. Singing is crucial   for males, which use their songs to attract mates and mark out their territory. The changing songs could play a role in eventually causing the city birds and the forest dwellers to evolve into separate species. The findings could also help explain why usually only a few bird species thrive in cities. By contrast, many more bird species tend to be found in forests and other undeveloped habitats. Studies like this could eventually help researchers predict how urbanization   will affect birds, using this knowledge could help urban planners design more pleasant city environments, study co-author Slabbe-koorn said. "You can make a city much more livable if you pay attention to where the noise goes and where the people go," he said. "It would be easy to fit the birds into those models." Then cities wouldn't be strictly for the people, but also for the birds. Males usually sing their songs to   _  .
evolve into another species attract mates and mark out their living area attract city people's attention and make them supply food allow themselves to be heard over the noises of the city
Question: A new study says that birds living in big cities sing shorter, faster songs that are higher-pitched   than those sung by their brothers in the forests. The researchers think that the birds adjust their songs to allow themselves to be heard over the noises of the city, especially the low traffic noise. To study how urban birds song, Hans Slabbekoorn and Ardie den Bore, biologists at the University of Leiden in Netherlands, travelled around Europe and recorded bird songs in ten major cities and in nearby forests. The species they focused on, the great tit  , is widespread across Eurasia and one of the few types of birds that  _ in big cities. Singing is crucial   for males, which use their songs to attract mates and mark out their territory. The changing songs could play a role in eventually causing the city birds and the forest dwellers to evolve into separate species. The findings could also help explain why usually only a few bird species thrive in cities. By contrast, many more bird species tend to be found in forests and other undeveloped habitats. Studies like this could eventually help researchers predict how urbanization   will affect birds, using this knowledge could help urban planners design more pleasant city environments, study co-author Slabbe-koorn said. "You can make a city much more livable if you pay attention to where the noise goes and where the people go," he said. "It would be easy to fit the birds into those models." Then cities wouldn't be strictly for the people, but also for the birds. Males usually sing their songs to   _  .
A. evolve into another species
B. attract mates and mark out their living area
C. attract city people's attention and make them supply food
D. allow themselves to be heard over the noises of the city
Answer: