近年来，许多国家大量的农村人口向城市迁移，其原因之一（　　）

A．城市污染比较少

B．城市有较多的就业机会

C．城市地多人多

D．城市的环境优美、空气清新

It’s easy to _________ this camera , while putting the pieces together is quite difficult, so you must keep the directions well.

A． take over

B． take apart

C． take down

D． take up