动物迁徙是动物主动适应环境的表现，其意义是

①有利于生殖②有利于获取足够的食物③有利于获得适宜的生存环境[ ]

A．①②    

B．③    

C．①    

D．①②③

Once air pollution       in most cities, it is hard      .

A．rises; dealing with

B．arouses; to be dealt with

C．raises; dealt with

D．arises; to deal with