答案:B
While every possible means __________ to prevent the river pollution, the river in my hometown has still not been restored to its past beauty.
A.is tried
B.are tried
C.has been tried
D.have been tried
冻融分选作用是由于石块和土的导热性能不同,冻结速度也各不一样。碎石导热率大,先冻结,水向石块附近迁移并在其附近形成冰,水形成冰后体积膨胀,使碎石产生移动,这样粗的物质和细的物质产生分离。经冻融分选作用,泥土和小的岩屑集中在中间,大的岩块被排挤到周边,呈多边形或近圆形,好像有人有意将石头摆成一圈,这种地貌叫作石环。读图回答下题。
以下地区最易出现石环地貌的是()
A.青藏高原地区
B.巴西高原
C.南极洲
D.亚马孙平原